Early identification in transactional buffered memory

ABSTRACT

A sequence of read returns are to be sent to a host device over a transactional buffered memory interface, where the sequence includes at least a first read return to a first read request and a second read return to a second read request. A tracker identifier of the second read return is encoded in the first read return and the first read return is sent with the tracker identifier of the second read return to the host device. The second read return is sent to the host device after the first read return is sent.

FIELD

The present disclosure relates in general to the field of computingarchitectures, and more specifically, to buffered memory protocols.

BACKGROUND

Advances in semi-conductor processing and logic design have permitted anincrease in the amount of logic that may be present on integratedcircuit devices. As a corollary, computer system configurations haveevolved from a single or multiple integrated circuits in a system tomultiple cores, multiple hardware threads, and multiple logicalprocessors present on individual integrated circuits, as well as otherinterfaces integrated within such processors. A processor or integratedcircuit typically comprises a single physical processor die, where theprocessor die may include any number of cores, hardware threads, logicalprocessors, interfaces, memory, controller hubs, etc.

As a result of the greater ability to fit more processing power insmaller packages, smaller computing devices have increased inpopularity. Smartphones, tablets, ultrathin notebooks, and other userequipment have grown exponentially. However, these smaller devices arereliant on servers both for data storage and complex processing thatexceeds the form factor. Consequently, the demand in thehigh-performance computing market (i.e. server space) has alsoincreased. For instance, in modern servers, there is typically not onlya single processor with multiple cores, but also multiple physicalprocessors (also referred to as multiple sockets) to increase thecomputing power. But as the processing power grows along with the numberof devices in a computing system, the communication between sockets andother devices becomes more critical.

In fact, interconnects have grown from more traditional multi-drop busesthat primarily handled electrical communications to full blowninterconnect architectures that facilitate fast communication.Unfortunately, as the demand for future processors to consume at evenhigher-rates corresponding demand is placed on the capabilities ofexisting interconnect architectures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified block diagram of a system including aserial point-to-point interconnect to connect I/O devices in a computersystem in accordance with one embodiment;

FIG. 2 illustrates a simplified block diagram of a layered protocolstack in accordance with one embodiment;

FIG. 3 illustrates an embodiment of a transaction descriptor.

FIG. 4 illustrates an embodiment of a serial point-to-point link.

FIG. 5 illustrates embodiments of potential High PerformanceInterconnect (HPI) system configurations.

FIG. 6 illustrates an embodiment of a layered protocol stack associatedwith HPI.

FIG. 7 illustrates a representation of an example state machine.

FIG. 8 illustrates a representation of an example flit sent over anexample twenty-lane data link.

FIG. 9 illustrates a representation of an example flit sent over anexample eight-lane data link.

FIG. 10 illustrates a representation of an example multi-slot flit.

FIG. 11 illustrates a representation of an example flit sent over anexample eight-lane data link.

FIG. 12 illustrates a representation of an example multi-layer flitincluding a debug message.

FIG. 13 illustrates a representation of a system utilizing a bufferedmemory protocol.

FIG. 14 illustrates a representation of a buffer device supporting abuffered memory protocol.

FIGS. 15A-15C illustrate representations of example flits in a bufferedmemory protocol.

FIG. 16 is a diagram illustrating communication between an examplebuffer device and a host device, in accordance with at least oneembodiment.

FIG. 17 is a diagram illustrating early communication of a read responsetracker identifier from a buffer device to a host device, in accordancewith at least one embodiment.

FIG. 18 illustrates an embodiment of a block for a computing systemincluding multiple processor sockets.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth,such as examples of specific types of processors and systemconfigurations, specific hardware structures, specific architectural andmicro architectural details, specific register configurations, specificinstruction types, specific system components, specific processorpipeline stages, specific interconnect layers, specificpacket/transaction configurations, specific transaction names, specificprotocol exchanges, specific link widths, specific implementations, andoperation etc. in order to provide a thorough understanding of thepresent invention. It may be apparent, however, to one skilled in theart that these specific details need not necessarily be employed topractice the subject matter of the present disclosure. In otherinstances, well detailed description of known components or methods hasbeen avoided, such as specific and alternative processor architectures,specific logic circuits/code for described algorithms, specific firmwarecode, low-level interconnect operation, specific logic configurations,specific manufacturing techniques and materials, specific compilerimplementations, specific expression of algorithms in code, specificpower down and gating techniques/logic and other specific operationaldetails of computer system in order to avoid unnecessarily obscuring thepresent disclosure.

Although the following embodiments may be described with reference toenergy conservation, energy efficiency, processing efficiency, and so onin specific integrated circuits, such as in computing platforms ormicroprocessors, other embodiments are applicable to other types ofintegrated circuits and logic devices. Similar techniques and teachingsof embodiments described herein may be applied to other types ofcircuits or semiconductor devices that may also benefit from suchfeatures. For example, the disclosed embodiments are not limited toserver computer system, desktop computer systems, laptops, Ultrabooks™,but may be also used in other devices, such as handheld devices,smartphones, tablets, other thin notebooks, systems on a chip (SOC)devices, and embedded applications. Some examples of handheld devicesinclude cellular phones, Internet protocol devices, digital cameras,personal digital assistants (PDAs), and handheld PCs. Here, similartechniques for a high-performance interconnect may be applied toincrease performance (or even save power) in a low power interconnect.Embedded applications typically include a microcontroller, a digitalsignal processor (DSP), a system on a chip, network computers (NetPC),set-top boxes, network hubs, wide area network (WAN) switches, or anyother system that can perform the functions and operations taught below.Moreover, the apparatus', methods, and systems described herein are notlimited to physical computing devices, but may also relate to softwareoptimizations for energy conservation and efficiency. As may becomereadily apparent in the description below, the embodiments of methods,apparatus', and systems described herein (whether in reference tohardware, firmware, software, or a combination thereof) may beconsidered vital to a “green technology” future balanced withperformance considerations.

As computing systems are advancing, the components therein are becomingmore complex. The interconnect architecture to couple and communicatebetween the components has also increased in complexity to ensurebandwidth demand is met for optimal component operation. Furthermore,different market segments demand different aspects of interconnectarchitectures to suit the respective market. For example, serversrequire higher performance, while the mobile ecosystem is sometimes ableto sacrifice overall performance for power savings. Yet, it is asingular purpose of most fabrics to provide highest possible performancewith maximum power saving. Further, a variety of different interconnectscan potentially benefit from subject matter described herein.

The Peripheral Component Interconnect (PCI) Express (PCIe) interconnectfabric architecture and QuickPath Interconnect (QPI) fabricarchitecture, among other examples, can potentially be improvedaccording to one or more principles described herein, among otherexamples. For instance, a primary goal of PCIe is to enable componentsand devices from different vendors to inter-operate in an openarchitecture, spanning multiple market segments; Clients (Desktops andMobile), Servers (Standard and Enterprise), and Embedded andCommunication devices. PCI Express is a high performance, generalpurpose I/O interconnect defined for a wide variety of future computingand communication platforms. Some PCI attributes, such as its usagemodel, load-store architecture, and software interfaces, have beenmaintained through its revisions, whereas previous parallel busimplementations have been replaced by a highly scalable, fully serialinterface. The more recent versions of PCI Express take advantage ofadvances in point-to-point interconnects, Switch-based technology, andpacketized protocol to deliver new levels of performance and features.Power Management, Quality Of Service (QoS), Hot-Plug/Hot-Swap support,Data Integrity, and Error Handling are among some of the advancedfeatures supported by PCI Express. Although the primary discussionherein is in reference to a new high-performance interconnect (HPI)architecture, aspects of the invention described herein may be appliedto other interconnect architectures, such as a PCIe-compliantarchitecture, a QPI-compliant architecture, a MIPI compliantarchitecture, a high-performance architecture, or other knowninterconnect architecture.

Referring to FIG. 1, an embodiment of a fabric composed ofpoint-to-point Links that interconnect a set of components isillustrated. System 100 includes processor 105 and system memory 110coupled to controller hub 115. Processor 105 can include any processingelement, such as a microprocessor, a host processor, an embeddedprocessor, a co-processor, or other processor. Processor 105 is coupledto controller hub 115 through front-side bus (FSB) 106. In oneembodiment, FSB 106 is a serial point-to-point interconnect as describedbelow. In another embodiment, link 106 includes a serial, differentialinterconnect architecture that is compliant with different interconnectstandard.

System memory 110 includes any memory device, such as random accessmemory (RAM), non-volatile (NV) memory, or other memory accessible bydevices in system 100. System memory 110 is coupled to controller hub115 through memory interface 116. Examples of a memory interface includea double-data rate (DDR) memory interface, a dual-channel DDR memoryinterface, and a dynamic RAM (DRAM) memory interface.

In one embodiment, controller hub 115 can include a root hub, rootcomplex, or root controller, such as in a PCIe interconnectionhierarchy. Examples of controller hub 115 include a chipset, a memorycontroller hub (MCH), a northbridge, an interconnect controller hub(ICH) a southbridge, and a root controller/hub. Often the term chipsetrefers to two physically separate controller hubs, e.g., a memorycontroller hub (MCH) coupled to an interconnect controller hub (ICH).Note that current systems often include the MCH integrated withprocessor 105, while controller 115 is to communicate with I/O devices,in a similar manner as described below. In some embodiments,peer-to-peer routing is optionally supported through root complex 115.

Here, controller hub 115 is coupled to switch/bridge 120 through seriallink 119. Input/output modules 117 and 121, which may also be referredto as interfaces/ports 117 and 121, can include/implement a layeredprotocol stack to provide communication between controller hub 115 andswitch 120. In one embodiment, multiple devices are capable of beingcoupled to switch 120.

Switch/bridge 120 routes packets/messages from device 125 upstream, i.e.up a hierarchy towards a root complex, to controller hub 115 anddownstream, i.e. down a hierarchy away from a root controller, fromprocessor 105 or system memory 110 to device 125. Switch 120, in oneembodiment, is referred to as a logical assembly of multiple virtualPCI-to-PCI bridge devices. Device 125 includes any internal or externaldevice or component to be coupled to an electronic system, such as anI/O device, a Network Interface Controller (NIC), an add-in card, anaudio processor, a network processor, a hard-drive, a storage device, aCD/DVD ROM, a monitor, a printer, a mouse, a keyboard, a router, aportable storage device, a Firewire device, a Universal Serial Bus (USB)device, a scanner, and other input/output devices. Often in the PCIevernacular, such as device, is referred to as an endpoint. Although notspecifically shown, device 125 may include a bridge (e.g., a PCIe toPCI/PCI-X bridge) to support legacy or other versions of devices orinterconnect fabrics supported by such devices.

Graphics accelerator 130 can also be coupled to controller hub 115through serial link 132. In one embodiment, graphics accelerator 130 iscoupled to an MCH, which is coupled to an ICH. Switch 120, andaccordingly I/O device 125, is then coupled to the ICH. I/O modules 131and 118 are also to implement a layered protocol stack to communicatebetween graphics accelerator 130 and controller hub 115. Similar to theMCH discussion above, a graphics controller or the graphics accelerator130 itself may be integrated in processor 105.

Turning to FIG. 2 an embodiment of a layered protocol stack isillustrated. Layered protocol stack 200 can includes any form of alayered communication stack, such as a QPI stack, a PCIe stack, a nextgeneration high performance computing interconnect (HPI) stack, or otherlayered stack. In one embodiment, protocol stack 200 can includetransaction layer 205, link layer 210, and physical layer 220. Aninterface, such as interfaces 117, 118, 121, 122, 126, and 131 in FIG.1, may be represented as communication protocol stack 200.Representation as a communication protocol stack may also be referred toas a module or interface implementing/including a protocol stack.

Packets can be used to communicate information between components.Packets can be formed in the Transaction Layer 205 and Data Link Layer210 to carry the information from the transmitting component to thereceiving component. As the transmitted packets flow through the otherlayers, they are extended with additional information used to handlepackets at those layers. At the receiving side the reverse processoccurs and packets get transformed from their Physical Layer 220representation to the Data Link Layer 210 representation and finally(for Transaction Layer Packets) to the form that can be processed by theTransaction Layer 205 of the receiving device.

In one embodiment, transaction layer 205 can provide an interfacebetween a device's processing core and the interconnect architecture,such as Data Link Layer 210 and Physical Layer 220. In this regard, aprimary responsibility of the transaction layer 205 can include theassembly and disassembly of packets (i.e., transaction layer packets, orTLPs). The translation layer 205 can also manage credit-based flowcontrol for TLPs. In some implementations, split transactions can beutilized, i.e., transactions with request and response separated bytime, allowing a link to carry other traffic while the target devicegathers data for the response, among other examples.

Credit-based flow control can be used to realize virtual channels andnetworks utilizing the interconnect fabric. In one example, a device canadvertise an initial amount of credits for each of the receive buffersin Transaction Layer 205. An external device at the opposite end of thelink, such as controller hub 115 in FIG. 1, can count the number ofcredits consumed by each TLP. A transaction may be transmitted if thetransaction does not exceed a credit limit. Upon receiving a response anamount of credit is restored. One example of an advantage of such acredit scheme is that the latency of credit return does not affectperformance, provided that the credit limit is not encountered, amongother potential advantages.

In one embodiment, four transaction address spaces can include aconfiguration address space, a memory address space, an input/outputaddress space, and a message address space. Memory space transactionsinclude one or more of read requests and write requests to transfer datato/from a memory-mapped location. In one embodiment, memory spacetransactions are capable of using two different address formats, e.g., ashort address format, such as a 32-bit address, or a long addressformat, such as 64-bit address. Configuration space transactions can beused to access configuration space of various devices connected to theinterconnect. Transactions to the configuration space can include readrequests and write requests. Message space transactions (or, simplymessages) can also be defined to support in-band communication betweeninterconnect agents. Therefore, in one example embodiment, transactionlayer 205 can assemble packet header/payload 206.

Quickly referring to FIG. 3, an example embodiment of a transactionlayer packet descriptor is illustrated. In one embodiment, transactiondescriptor 300 can be a mechanism for carrying transaction information.In this regard, transaction descriptor 300 supports identification oftransactions in a system. Other potential uses include trackingmodifications of default transaction ordering and association oftransaction with channels. For instance, transaction descriptor 300 caninclude global identifier field 302, attributes field 304 and channelidentifier field 306. In the illustrated example, global identifierfield 302 is depicted comprising local transaction identifier field 308and source identifier field 310. In one embodiment, global transactionidentifier 302 is unique for all outstanding requests.

According to one implementation, local transaction identifier field 308is a field generated by a requesting agent, and can be unique for alloutstanding requests that require a completion for that requestingagent. Furthermore, in this example, source identifier 310 uniquelyidentifies the requestor agent within an interconnect hierarchy.Accordingly, together with source ID 310, local transaction identifier308 field provides global identification of a transaction within ahierarchy domain.

Attributes field 304 specifies characteristics and relationships of thetransaction. In this regard, attributes field 304 is potentially used toprovide additional information that allows modification of the defaulthandling of transactions. In one embodiment, attributes field 304includes priority field 312, reserved field 314, ordering field 316, andno-snoop field 318. Here, priority sub-field 312 may be modified by aninitiator to assign a priority to the transaction. Reserved attributefield 314 is left reserved for future, or vendor-defined usage. Possibleusage models using priority or security attributes may be implementedusing the reserved attribute field.

In this example, ordering attribute field 316 is used to supply optionalinformation conveying the type of ordering that may modify defaultordering rules. According to one example implementation, an orderingattribute of “0” denotes default ordering rules are to apply, wherein anordering attribute of “1” denotes relaxed ordering, wherein writes canpass writes in the same direction, and read completions can pass writesin the same direction. Snoop attribute field 318 is utilized todetermine if transactions are snooped. As shown, channel ID Field 306identifies a channel that a transaction is associated with.

Returning to the discussion of FIG. 2, a Link layer 210, also referredto as data link layer 210, can act as an intermediate stage betweentransaction layer 205 and the physical layer 220. In one embodiment, aresponsibility of the data link layer 210 is providing a reliablemechanism for exchanging Transaction Layer Packets (TLPs) between twocomponents on a link. One side of the Data Link Layer 210 accepts TLPsassembled by the Transaction Layer 205, applies packet sequenceidentifier 211, i.e. an identification number or packet number,calculates and applies an error detection code, i.e. CRC 212, andsubmits the modified TLPs to the Physical Layer 220 for transmissionacross a physical to an external device.

In one example, physical layer 220 includes logical sub block 221 andelectrical sub-block 222 to physically transmit a packet to an externaldevice. Here, logical sub-block 221 is responsible for the “digital”functions of Physical Layer 221. In this regard, the logical sub-blockcan include a transmit section to prepare outgoing information fortransmission by physical sub-block 222, and a receiver section toidentify and prepare received information before passing it to the LinkLayer 210.

Physical block 222 includes a transmitter and a receiver. Thetransmitter is supplied by logical sub-block 221 with symbols, which thetransmitter serializes and transmits onto to an external device. Thereceiver is supplied with serialized symbols from an external device andtransforms the received signals into a bit-stream. The bit-stream isde-serialized and supplied to logical sub-block 221. In one exampleembodiment, an 8b/10b transmission code is employed, where ten-bitsymbols are transmitted/received. Here, special symbols are used toframe a packet with frames 223. In addition, in one example, thereceiver also provides a symbol clock recovered from the incoming serialstream.

As stated above, although transaction layer 205, link layer 210, andphysical layer 220 are discussed in reference to a specific embodimentof a protocol stack (such as a PCIe protocol stack), a layered protocolstack is not so limited. In fact, any layered protocol may beincluded/implemented and adopt features discussed herein. As an example,a port/interface that is represented as a layered protocol can include:(1) a first layer to assemble packets, i.e. a transaction layer; asecond layer to sequence packets, i.e. a link layer; and a third layerto transmit the packets, i.e. a physical layer. As a specific example, ahigh performance interconnect layered protocol, as described herein, isutilized.

Referring next to FIG. 4, an example embodiment of a serial point topoint fabric is illustrated. A serial point-to-point link can includeany transmission path for transmitting serial data. In the embodimentshown, a link can include two, low-voltage, differentially driven signalpairs: a transmit pair 406/411 and a receive pair 412/407. Accordingly,device 405 includes transmission logic 406 to transmit data to device410 and receiving logic 407 to receive data from device 410. In otherwords, two transmitting paths, i.e. paths 416 and 417, and two receivingpaths, i.e. paths 418 and 419, are included in some implementations of alink.

A transmission path refers to any path for transmitting data, such as atransmission line, a copper line, an optical line, a wirelesscommunication channel, an infrared communication link, or othercommunication path. A connection between two devices, such as device 405and device 410, is referred to as a link, such as link 415. A link maysupport one lane—each lane representing a set of differential signalpairs (one pair for transmission, one pair for reception). To scalebandwidth, a link may aggregate multiple lanes denoted by xN, where N isany supported link width, such as 1, 2, 4, 8, 12, 16, 32, 64, or wider.

A differential pair can refer to two transmission paths, such as lines416 and 417, to transmit differential signals. As an example, when line416 toggles from a low voltage level to a high voltage level, i.e. arising edge, line 417 drives from a high logic level to a low logiclevel, i.e. a falling edge. Differential signals potentially demonstratebetter electrical characteristics, such as better signal integrity, i.e.cross-coupling, voltage overshoot/undershoot, ringing, among otherexample advantages. This allows for a better timing window, whichenables faster transmission frequencies.

In one embodiment, a new High Performance Interconnect (HPI) isprovided. HPI can include a next-generation cache-coherent, link-basedinterconnect. As one example, HPI may be utilized in high performancecomputing platforms, such as workstations or servers, including insystems where PCIe or another interconnect protocol is typically used toconnect processors, accelerators, I/O devices, and the like. However,HPI is not so limited. Instead, HPI may be utilized in any of thesystems or platforms described herein. Furthermore, the individual ideasdeveloped may be applied to other interconnects and platforms, such asPCIe, MIPI, QPI, etc.

To support multiple devices, in one example implementation, HPI caninclude an Instruction Set Architecture (ISA) agnostic (i.e. HPI is ableto be implemented in multiple different devices). In another scenario,HPI may also be utilized to connect high performance I/O devices, notjust processors or accelerators. For example, a high performance PCIedevice may be coupled to HPI through an appropriate translation bridge(i.e. HPI to PCIe). Moreover, the HPI links may be utilized by many HPIbased devices, such as processors, in various ways (e.g. stars, rings,meshes, etc.). FIG. 5 illustrates example implementations of multiplepotential multi-socket configurations. A two-socket configuration 505,as depicted, can include two HPI links; however, in otherimplementations, one HPI link may be utilized. For larger topologies,any configuration may be utilized as long as an identifier (ID) isassignable and there is some form of virtual path, among otheradditional or substitute features. As shown, in one example, a foursocket configuration 510 has an HPI link from each processor to another.But in the eight socket implementation shown in configuration 515, notevery socket is directly connected to each other through an HPI link.However, if a virtual path or channel exists between the processors, theconfiguration is supported. A range of supported processors includes2-32 in a native domain. Higher numbers of processors may be reachedthrough use of multiple domains or other interconnects between nodecontrollers, among other examples.

The HPI architecture includes a definition of a layered protocolarchitecture, including in some examples, protocol layers (coherent,non-coherent, and, optionally, other memory based protocols), a routinglayer, a link layer, and a physical layer. Furthermore, HPI can furtherinclude enhancements related to power managers (such as power controlunits (PCUs)), design for test and debug (DFT), fault handling,registers, security, among other examples. FIG. 5 illustrates anembodiment of an example HPI layered protocol stack. In someimplementations, at least some of the layers illustrated in FIG. 5 maybe optional. Each layer deals with its own level of granularity orquantum of information (the protocol layer 605 a,b with packets 630,link layer 610 a,b with flits 635, and physical layer 605 a,b with phits640). Note that a packet, in some embodiments, may include partialflits, a single flit, or multiple flits based on the implementation.

As a first example, a width of a phit 640 includes a 1 to 1 mapping oflink width to bits (e.g. 20 bit link width includes a phit of 20 bits,etc.). Flits may have a greater size, such as 184, 192, or 200 bits.Note that if phit 640 is 20 bits wide and the size of flit 635 is 184bits then it takes a fractional number of phits 640 to transmit one flit635 (e.g. 9.2 phits at 20 bits to transmit an 184 bit flit 635 or 9.6 at20 bits to transmit a 192 bit flit, among other examples). Note thatwidths of the fundamental link at the physical layer may vary. Forexample, the number of lanes per direction may include 2, 4, 6, 8, 10,12, 14, 16, 18, 20, 22, 24, etc. In one embodiment, link layer 610 a,bis capable of embedding multiple pieces of different transactions in asingle flit, and one or multiple headers (e.g. 1, 2, 3, 4) may beembedded within the flit. In one example, HPI splits the headers intocorresponding slots to enable multiple messages in the flit destined fordifferent nodes.

Physical layer 605 a,b, in one embodiment, can be responsible for thefast transfer of information on the physical medium (electrical oroptical etc.). The physical link can be point-to-point between two Linklayer entities, such as layer 605 a and 605 b. The Link layer 610 a,bcan abstract the Physical layer 605 a,b from the upper layers andprovides the capability to reliably transfer data (as well as requests)and manage flow control between two directly connected entities. TheLink Layer can also be responsible for virtualizing the physical channelinto multiple virtual channels and message classes. The Protocol layer620 a,b relies on the Link layer 610 a,b to map protocol messages intothe appropriate message classes and virtual channels before handing themto the Physical layer 605 a,b for transfer across the physical links.Link layer 610 a,b may support multiple messages, such as a request,snoop, response, writeback, non-coherent data, among other examples.

The Physical layer 605 a,b (or PHY) of HPI can be implemented above theelectrical layer (i.e. electrical conductors connecting two components)and below the link layer 610 a,b, as illustrated in FIG. 6. The Physicallayer and corresponding logic can reside on each agent and connects thelink layers on two agents (A and B) separated from each other (e.g. ondevices on either side of a link). The local and remote electricallayers are connected by physical media (e.g. wires, conductors, optical,etc.). The Physical layer 605 a,b, in one embodiment, has two majorphases, initialization and operation. During initialization, theconnection is opaque to the link layer and signaling may involve acombination of timed states and handshake events. During operation, theconnection is transparent to the link layer and signaling is at a speed,with all lanes operating together as a single link. During the operationphase, the Physical layer transports flits from agent A to agent B andfrom agent B to agent A. The connection is also referred to as a linkand abstracts some physical aspects including media, width and speedfrom the link layers while exchanging flits and control/status ofcurrent configuration (e.g. width) with the link layer. Theinitialization phase includes minor phases e.g. Polling, Configuration.The operation phase also includes minor phases (e.g. link powermanagement states).

In one embodiment, Link layer 610 a,b can be implemented so as toprovide reliable data transfer between two protocol or routing entities.The Link layer can abstract Physical layer 605 a,b from the Protocollayer 620 a,b, and can be responsible for the flow control between twoprotocol agents (A, B), and provide virtual channel services to theProtocol layer (Message Classes) and Routing layer (Virtual Networks).The interface between the Protocol layer 620 a,b and the Link Layer 610a,b can typically be at the packet level. In one embodiment, thesmallest transfer unit at the Link Layer is referred to as a flit whicha specified number of bits, such as 192 bits or some other denomination.The Link Layer 610 a,b relies on the Physical layer 605 a,b to frame thePhysical layer's 605 a,b unit of transfer (phit) into the Link Layer's610 a,b unit of transfer (flit). In addition, the Link Layer 610 a,b maybe logically broken into two parts, a sender and a receiver. Asender/receiver pair on one entity may be connected to a receiver/senderpair on another entity. Flow Control is often performed on both a flitand a packet basis. Error detection and correction is also potentiallyperformed on a flit level basis.

In one embodiment, Routing layer 615 a,b can provide a flexible anddistributed method to route HPI transactions from a source to adestination. The scheme is flexible since routing algorithms formultiple topologies may be specified through programmable routing tablesat each router (the programming in one embodiment is performed byfirmware, software, or a combination thereof). The routing functionalitymay be distributed; the routing may be done through a series of routingsteps, with each routing step being defined through a lookup of a tableat either the source, intermediate, or destination routers. The lookupat a source may be used to inject a HPI packet into the HPI fabric. Thelookup at an intermediate router may be used to route an HPI packet froman input port to an output port. The lookup at a destination port may beused to target the destination HPI protocol agent. Note that the Routinglayer, in some implementations, can be thin since the routing tables,and, hence the routing algorithms, are not specifically defined byspecification. This allows for flexibility and a variety of usagemodels, including flexible platform architectural topologies to bedefined by the system implementation. The Routing layer 615 a,b relieson the Link layer 610 a,b for providing the use of up to three (or more)virtual networks (VNs)—in one example, two deadlock-free VNs, VN0 andVN1 with several message classes defined in each virtual network. Ashared adaptive virtual network (VNA) may be defined in the Link layer,but this adaptive network may not be exposed directly in routingconcepts, since each message class and virtual network may havededicated resources and guaranteed forward progress, among otherfeatures and examples.

In one embodiment, HPI can include a Coherence Protocol layer 620 a,b tosupport agents caching lines of data from memory. An agent wishing tocache memory data may use the coherence protocol to read the line ofdata to load into its cache. An agent wishing to modify a line of datain its cache may use the coherence protocol to acquire ownership of theline before modifying the data. After modifying a line, an agent mayfollow protocol requirements of keeping it in its cache until it eitherwrites the line back to memory or includes the line in a response to anexternal request. Lastly, an agent may fulfill external requests toinvalidate a line in its cache. The protocol ensures coherency of thedata by dictating the rules all caching agents may follow. It alsoprovides the means for agents without caches to coherently read andwrite memory data.

Two conditions may be enforced to support transactions utilizing the HPICoherence Protocol. First, the protocol can maintain data consistency,as an example, on a per-address basis, among data in agents' caches andbetween those data and the data in memory. Informally, data consistencymay refer to each valid line of data in an agent's cache representing amost up-to-date value of the data and data transmitted in a coherenceprotocol packet can represent the most up-to-date value of the data atthe time it was sent. When no valid copy of the data exists in caches orin transmission, the protocol may ensure the most up-to-date value ofthe data resides in memory. Second, the protocol can providewell-defined commitment points for requests. Commitment points for readsmay indicate when the data is usable; and for writes they may indicatewhen the written data is globally observable and will be loaded bysubsequent reads. The protocol may support these commitment points forboth cacheable and uncacheable (UC) requests in the coherent memoryspace.

In some implementations, HPI can utilize an embedded clock. A clocksignal can be embedded in data transmitted using the interconnect. Withthe clock signal embedded in the data, distinct and dedicated clocklanes can be omitted. This can be useful, for instance, as it can allowmore pins of a device to be dedicated to data transfer, particularly insystems where space for pins is at a premium.

A link can be established between two agents on either side of aninterconnect. An agent sending data can be a local agent and the agentreceiving the data can be a remote agent. State machines can be employedby both agents to manage various aspects of the link. In one embodiment,the Physical layer data path can transmit flits from the link layer tothe electrical front-end. The control path, in one implementation,includes a state machine (also referred to as a link training statemachine or the similar). The state machine's actions and exits fromstates may depend on internal signals, timers, external signals or otherinformation. In fact, some of the states, such as a few initializationstates, may have timers to provide a timeout value to exit a state. Notethat detect, in some embodiments, refers to detecting an event on bothlegs of a lane; but not necessarily simultaneously. However, in otherembodiments, detect refers to detection of an event by an agent ofreference. Debounce, as one example, refers to sustained assertion of asignal. In one embodiment, HPI supports operation in the event ofnon-function lanes. Here, lanes may be dropped at specific states.

States defined in the state machine can include reset states,initialization states, and operational states, among other categoriesand subcategories. In one example, some initialization states can have asecondary timer which is used to exit the state on a timeout(essentially an abort due to failure to make progress in the state). Anabort may include updating of registers, such as status register. Somestates can also have primary timer(s) which are used to time the primaryfunctions in the state. Other states can be defined such that internalor external signals (such as handshake protocols) drive transition fromthe state to another state, among other examples.

A state machine may also support debug through single step, freeze oninitialization abort and use of testers. Here, state exits can bepostponed/held until the debug software is ready. In some instance, theexit can be postponed/held until the secondary timeout. Actions andexits, in one embodiment, can be based on exchange of trainingsequences. In one embodiment, the link state machine is to run in thelocal agent clock domain and transition from one state to the next is tocoincide with a transmitter training sequence boundary. Status registersmay be utilized to reflect the current state.

FIG. 7 illustrates a representation of at least a portion of a statemachine used by agents in one example implementation of HPI. It shouldbe appreciated that the states included in the state table of FIG. 7include a non-exhaustive listing of possible states. For instance, sometransitions are omitted to simplify the diagram. Also, some states maybe combined, split, or omitted, while others might be added. Such statescan include:

Event reset state: entered on a warm or cold reset event. Restoresdefault values. Initialize counters (e.g., sync counters). May exit toanother state, such as another reset state.

Timed reset state: timed state for in-band reset. May drive a predefinedelectrical ordered set (EOS) so remote receivers are capable ofdetecting the EOS and entering the timed reset as well. Receiver haslanes holding electrical settings. May exit to an agent to calibratereset state.

Calibrate reset state: calibration without signaling on the lane (e.g.receiver calibration state) or turning drivers off. May be apredetermined amount of time in the state based on a timer. May set anoperational speed. May act as a wait state when a port is not enabled.May include minimum residency time. Receiver conditioning or staggeringoff may occur based on design. May exit to a receiver detect state aftera timeout and/or completion of calibration.

Receiver detect state: detect presence of a receiver on lane(s). Maylook for receiver termination (e.g. receiver pulldown insertion). Mayexit to calibrate reset state upon a specified value being set or whenanother specified value is not set. May exit to transmitter calibratestate if a receiver is detected or a timeout is reached.

Transmitter calibrate state: for transmitter calibrations. May be atimed state allocated for transmitter calibrations. May includesignaling on a lane. May continuously drive an EOS, such as an electricidle exit ordered set (or EIEIOS). May exit to compliance state whendone calibrating or on expiration of a timer. May exit to transmitterdetect state if a counter has expired or a secondary timeout hasoccurred.

Transmitter detect state: qualifies valid signaling. May be a handshakestate where an agent completes actions and exits to a next state basedon remote agent signaling. Receiver may qualify valid signaling fromtransmitter. Receiver, in one embodiment, looks for a wake detect, andif debounced on one or more lanes looks for it on the other lanes.Transmitter drives a detect signal. May exit to a polling state inresponse to debounce being completed for all lanes and/or a timeout orif debounce on all lanes is not complete and there is a timeout. Here,one or more monitor lanes may be kept awake to debounce a wake signal.And if debounced then the other lanes are potentially debounced. Thiscan enable power savings in low power states.

Polling state: receiver adapts, locks on bits (e.g., by initializing adrift buffer), locks on bytes (e.g., by identifying symbol boundaries),and locks on training sequence boundaries (e.g., by locking to anelectrical ordered set (EOS) and/or a training sequence header). Lanesmay then be deskewed. Handshakes can also be completed to exit pollingto one of several potential states. For instance, an exit can be caused(by an agent) to any one of a Link Width state, Compliance state,Loopback Marker state, or Reset state. Handshaking can include thecompletion of various defined receiver actions or conditions and thesending of an acknowledge message (ACK) to trigger the exit to the statecorresponding to the completed set of actions and the ACK. For pollingdeskew, lane to lane skew at remote transmitter may be capped at a firstlength for top speed and a second length for slow speed. Deskew may beperformed in a slow mode as well as an operational mode. Receiver mayhave a specific maximum to deskew lane-to-lane skew, such as 8, 16, or32 intervals of skew. Receiver actions may also include latency fixingin some implementations. Receiver actions, in one embodiment, can becompleted on successful deskew of a valid lane map. A successfulhandshake can be achieved, in one example, when a number of consecutivetraining sequence headers are received with acknowledgements and anumber of training sequences with an acknowledge are transmitted afterthe receiver has completed its actions.

Compliance state: entered from the Polling state. An agent may be made aCompliance master or slave for validation purposes. The device (actingas master) can send a compliance pattern to another device (slave) andthe slave can loop back the pattern after re-timing it to its localclock (e.g., without undo of any polarity inversion or lane reversal).The compliance pattern can be used to characterize operation of theanalog front end on some subset of lanes when Loopback is not functionalor undesired. For instance, entering Loopback can be preconditioned onsuccessful byte lock, TS lock, deskew, latency testing, and othertesting reliant on the proper functioning of several piece of digitallogic. With bit lock accomplished, Compliance can be entered and may beutilized for jitter or noise investigation, debug, exploring a link,among other conditioning. In other words, Compliance can serve as analternative exit for Polling in the event the results of Polling do notallow direct entry into Loopback. The Compliance state can drive asupersequence with a transmitter from the master. Receiver looks for awake on a monitor lane, debounces the wake, drops bad lanes, adapts, andbit locks, etc. The slave transmitter can drive the compliance patternuntil its receiver actions are complete. Then loop-back is re-timed andnon-deskewed. Slave receiver does similar monitor and debounce, etc.actions. Exit may be to a reset state, such as a timed reset, or to aLoopback Pattern state to start the test, among other examples. In thecase of an exit to Loopback Pattern state, the master may be sent (e.g.,by a software controller) to the Loopback Pattern state to try out morespecialized patterns.

Agent Loopback Marker State: Loopback marker is an agent state butunlike other agent states master and slave actions and exits may bedifferent. Loopback slave may undo any polarity inversion and/or lanereversal but may not descramble or rescramble looped back bits.Acknowledgment exchange may not apply to slave since it is looping back.Since slave may deskew before looping back on symbol boundary, mastermay not be forced to re-bytelock or re-deskew but the master may re-locktraining sequence to avoid locking to some alias. Means to do this mayinclude re-seeding of LFSR, comparing TS and/or EIEOS or somecombination of these. The end of the SDS marks the end of loopback setupand the start of pattern generation, checking and counting.

Agent Loopback Pattern State (or Block Link state): Entered from eithera Loopback Marker state or Compliance state. In Loopback Pattern state,instead of control patterns, a master transmitter can send additionalspecialized patterns to supplement those looped-back in a Compliance orLoopback Pattern state. The receiver can receive specialized patterns inLoopback Pattern and check for errors in the received pattern. Fortransmitter adaptation both agents can be masters. For a predeterminedperiod, the transmitter can sends a pattern and a remote receiver cancompare this pattern and determine a figure of merit or metric for thereceived pattern which is recorded in a storage element, such as aregister. The comparison method and metric may be design dependent(e.g., BER with jitter injection). At the end of the period, both agentscan exit to Reset for the backchannel to examine the metric and set upthe next iteration of transmitter adaptation.

Link width state: agent communicates with the final lane map to remotetransmitter. Receiver receives the information and decodes. Receiver mayrecord a configured lane map in a structure after checkpoint of aprevious lane map value in a second structure. Receiver may also respondwith an acknowledge (“ACK”). May initiate an in-band reset. As oneexample, first state to initiate in-band reset. In one embodiment, exitto a next state, such as flit configuration state, is performed inresponse to the ACK. Further, prior to entering low power state, a resetsignal may also be generated if the frequency of a wake detect signaloccurrence drops below a specified value (e.g. 1 every number of unitintervals (UIs), such as 4K UI). Receiver may hold current and previouslane maps. Transmitter may use different groups of lanes based ontraining sequences having different values. Lane map may not modify somestatus registers in some embodiments.

Flitlock configuration state: entered by a transmitter but the state isconsidered exited (i.e. secondary timeout moot) when both transmitterand receiver have exited to a blocking link state or other link state.Transmitter exit to a link state, in one embodiment, includes start of adata sequence (SDS) and training sequence (TS) boundary after receivinga planetary alignment signal. Here, receiver exit may be based onreceiving an SDS from a remote transmitter. This state may be a bridgefrom agent to link state. Receiver identifies SDS. Receiver may exit toblocking link state (BLS) (or a control window) if SDS received after adescrambler is initialized. If a timeout occurs, exit may be to resetstate. Transmitter drives lanes with a configuration signal. Transmitterexit may be to reset, BLS, or other states based on conditions ortimeouts.

Transmitting Link State: a link state. Flits are sent to a remote agent.May be entered from a blocking link state and return to a blocking linkstate on an event, such as a timeout. Transmitter transmits flits.Receiver receives flits. May also exit to a low power link state. Insome implementations, transmitting link state (TLS) can be referred toas the L0 state.

Blocking Link State: a link state. Transmitter and receiver areoperating in a unified manner. May be a timed state during which thelink layer flits are held off while the Physical layer information iscommunicated to the remote agent. May exit to a low power link state (orother link state based on the design). A blocking link state (BLS), inone embodiment, periodically occurs. The period is referred to as a BLSinterval and may be timed, as well as may differ between slow speed andoperational speed. Note that the link layer may be periodically blockedfrom sending flits so that a Physical layer control sequence of a lengthmay be sent, such as during a transmitting link state or a partial widthtransmitting link state. In some implementations, blocking link state(BLS) can be referred to as a L0 control, or L0c, state.

Partial Width Transmitting Link State: Link state. May save power byentering a partial width state. In one embodiment asymmetric partialwidth refers to each direction of a two direction link having differentwidths, which may be supported in some designs. An example of aninitiator, such as a transmitter, sending a partial width indication toenter partial width transmitting link state is shown in the example ofFIG. 9. Here, a partial width indication is sent while transmitting on alink with a first width to transition the link to transmit at a second,new width. A mismatch may result in a reset. Note that speeds may not bealtered but width may be. Therefore, flits are potentially sent atdifferent widths. May be similar to a transmitting link state logically;yet, since there is a smaller width, it may take longer to transmitflits. May exit to other link states, such as a low power link statebased on certain received and sent messages or an exit of the partialwidth transmitting link state or a link blocking state based on otherevents. In one embodiment, a transmitter port may turn idle lanes off ina staggered manner to provide better signal integrity (i.e. noisemitigation) as shown in the timing diagram. Here, non-retry-able flits,such as Null flits, may be utilized during periods where the link widthis changing. A corresponding receiver may drop these null flits and turnidle lanes off in a staggered manner, as well as record the current andprevious lane maps in one or more structures. Note status and associatedstatus register may remain unaltered. In some implementations, partialwidth transmitting link state can be referred to as a partial L0, orL0p, state.

Exit Partial Width Transmitting Link State: exit the partial widthstate. May or may not use a blocking link state in some implementations.The transmitter initiates exit, in one embodiment, by sending partialwidth exit patterns on the idle lanes to train and deskew them. As oneexample, an exit pattern start with EIEOS, which is detected anddebounced to signal that the lane is ready to start the entry to a fulltransmitting link state, and may end with SDS or Fast Training Sequence(FTS) on idle lanes. Any failure during the exit sequence (receiveractions, such as deskew not completed prior to timeout) stops flittransfers to the link layer and asserts a reset, which is handled byresetting the link on the next blocking link state occurrence. The SDSmay also initialize the scrambler/descrambler on the lanes toappropriate values.

Low Power Link State: is a lower power state. In one embodiment, it islower power than the partial width link state, since signaling in thisembodiment is stopped on all lanes and in both directions. Transmittersmay use a blocking link state for requesting a low power link state.Here, receiver may decode the request and respond with an ACK or a NAK;otherwise reset may be triggered. In some implementations, low powerlink state can be referred to as a L1 state.

In some implementations, state transitions can be facilitated to allowstates to be bypassed, for instance, when state actions of the states,such as certain calibrations and configurations, have already beencompleted. Previous state results and configurations of a link can bestored and reused in subsequent initializations and configurations of alink. Rather than repeating such configurations and state actions,corresponding states can be bypassed. Traditional systems implementingstate bypasses, however, often implement complex designs and expensivevalidation escapes. Rather than using a traditional bypass, in oneexample, HPI can utilize short timers in certain states, such as wherethe state actions do not need to be repeated. This can potentially allowfor more uniform and synchronized state machine transitions among otherpotential advantages.

In one example, a software-based controller (e.g., through an externalcontrol point for the Physical layer) can enable a short timer for oneor more particular states. For instance, for a state for which actionshave already been performed and stored, the state can be short-timed tofacilitate a quick exit from the state to a next state. If, however, theprevious state action fails or cannot be applied within the short timerduration, a state exit can be performed. Further, the controller candisable the short timer, for instance, when the state actions should beperformed anew. A long, or default, timer can be set for each respectivestate. If configuration actions at the state cannot be completed withinthe long timer, a state exit can occur. The long timer can be set to areasonable duration so as to allow completion of the state actions. Theshort timer, in contrast, may be considerably shorter making it, in somecases, impossible to perform the state actions without reference back topreviously-performed state actions, among other examples.

In some instances, during initialization (or re-initialization) of alink, as agents progress through a state machine toward an operationallink state, one or more failures or state exits can occur that cause thestate to reset (e.g., to a reset or other state). In effect, theinitialization of the link can loop through one or more states withoutcompleting the initialization and entering a link state. In one example,a count can be maintained for the number of unproductive loops in statetransitions within the initialization of a link. For instance, each timean initialization returns to a reset state without reaching a link statea counter can be incremented. The counter can be reset for the link oncethe link successfully enters a link state. Such counters can bemaintained by agents on both sides of the link. Further, a threshold canbe set, for instance, by a software-based controller utilizing one ormore external control points. When the count of unproductive loops meets(or exceeds) the defined threshold initialization of the link can besuspended (e.g., set and held at or before the reset state). In someimplementations, in order to recommence initialization and release theinitialization from the suspended state, a software-based controller cantrigger a restart or re-initialization of the link. In some instances,the software-based tools can analyze the nature of the suspendedinitialize and perform diagnostics, set register values, and performother operations so as to guard against further looping of theinitialization. Indeed, in some implementations, a controller can set ahigher counter threshold or even override the counter, among otherexamples, in connection with restarting a suspended link initialization.

In some implementations of HPI, supersequences can be defined, eachsupersequence corresponding to a respective state or entry/exit to/fromthe respective state. A supersequence can include a repeating sequenceof data sets and symbols. The sequences can repeat, in some instances,until completion of a state or state transition, or communication of acorresponding event, among other examples. In some instances, therepeating sequence of a supersequence can repeat according to a definedfrequency, such as a defined number of unit intervals (UIs). A unitinterval (UI) can correspond to the interval of time for transmitting asingle bit on a lane of a link or system. In some implementations, therepeating sequence can begin with an electrically ordered set (EOS).Accordingly, an instance of the EOS can be expected to repeat inaccordance with the predefined frequency. Such ordered sets can beimplemented as defined 16 Byte codes that may be represented inhexadecimal format, among other examples. In one example, the EOS of asupersequence can be an EIEIOS. In one example, an EIEOS can resemble alow frequency clock signal (e.g., a predefined number of repeating FF00or FFF000 hexadecimal symbols, etc.). A predefined set of data canfollow the EOS, such as a predefined number of training sequences orother data. Such supersequences can be utilized in state transitionsincluding link state transitions as well as initialization, among otherexamples.

In some implementations of an interconnect, such as in QPI, terminationsof a serial data link can be brought on and off, such as when a link isreset or initialized. This approach can introduce complexity and timeinto the initialization of a link. In some implementations of HPI,terminations of the link can be maintained including during a reset orre-initialization of the link. Further, HPI can permit hot-plugging ofdevices. When another device is introduced, either through hot-pluggingor otherwise, the voltage characteristics of the lane on which the newremote agent is added will change. The local agent can sense thesechanges in the lane voltage to detect the presence of the remote agentand prompt initialization of the link. State machine states and timerscan be defined in the state machine to coordinate the detection,configuration, and initialization of a link without terminations.

In one implementation, HPI can support re-initialization on an in-bandreset without changing the termination values through the screening of alane by the receiving agent for incoming signaling. The signaling can beused to identify good lanes. As an example, the lane can be screened forany one of a set of pre-defined signals that are to be sent by atransmitter device to facilitate discovery and configuration of thelink. In one example, a supersequence can be defined corresponding toone or more initialization or re-initialization tasks. The pre-definedsequence can include an electric idle exit ordered set (EIEOS) followedby additional sequence data. In some instances, as each device on eitherside of a lane becomes active, the devices can begin sending asupersequence corresponding to a particular initialization state, etc.In one embodiment, two types of pin resets can be supported; power-on(or “cold”) reset and warm reset. A reset initiated by software ororiginating (in the Physical or another layer) on one agent may becommunicated in-band to the other agent. However, due to usage of anembedded clock, an in-band reset may be handled by communication toanother agent using an ordered set, such as a specific electricalordered set or EIOS.

The ordered set can be sent during initialization and a PHY controlsequence (or “blocking link state”) can be sent after initialization.The block link state can block the link layer from sending flits. Asanother example, link layer traffic may be blocked to send a few NULLflits which may be discarded at the receiver.

As introduced above, initialization, in one embodiment, can be doneinitially at slow speed followed by initialization at fast speed.Initialization at slow speed uses the default values for the registersand timers. Software then uses the slow speed link to setup theregisters, timers and electrical parameters and clears the calibrationsemaphores to pave the way for fast speed initialization. As oneexample, initialization can consist of such states or tasks as Reset,Detect, Polling, Compliance, and Configuration, among potentiallyothers.

In one example, a link layer blocking control sequence (i.e. a blockinglink state (BLS) or L0c state) can include a timed state during whichthe link layer flits are held off while the PHY information iscommunicated to the remote agent. Here, the transmitter and receiver maystart a block control sequence timer. And upon expiration of the timers,the transmitter and receiver can exit the blocking state and may takeother actions, such as exit to reset, exit to a different link state (orother state), including states that allow for the sending of flitsacross the link.

In one embodiment, link training can be provided and include the sendingof one or more of scrambled training sequences, ordered sets, andcontrol sequences, such as in connection with a defined supersequence. Atraining sequence symbol may include one or more of a header, reservedportions, a target latency, a pair number, a physical lane map codereference lanes or a group of lanes, and an initialization state. In oneembodiment, the header can be sent with a ACK or NAK, among otherexamples. As an example, training sequences may be sent as part ofsupersequences and may be scrambled.

In one embodiment, ordered sets and control sequences are not scrambledor staggered and are transmitted identically, simultaneously andcompletely on all lanes. A valid reception of an ordered set may includechecking of at least a portion of the ordered set (or entire ordered setfor partial ordered sets). Ordered sets may include an electricallyordered set (EOS), such as an Electrical Idle Ordered Set (EIOS) or anEIEOS. A supersequence may include a start of a data sequence (SDS) or aFast Training Sequence (FTS). Such sets and control supersequences canbe predefined and may have any pattern or hexadecimal representation, aswell as any length. For example, ordered sets and supersequences may bea length of 8 bytes, 16, bytes, or 32 bytes, etc. FTS, as an example,can additionally be utilized for fast bit lock during exit of a partialwidth transmitting link state. Note that the FTS definition may be perlane and may utilize a rotated version of the FTS.

Supersequences, in one embodiment, can include the insertion of an EOS,such as an EIEOS, in a training sequence stream. When signaling starts,lanes, in one implementation, power-on in a staggered manner. This mayresult, however, in initial supersequences being seen truncated at thereceiver on some lanes. Supersequences can be repeated however overshort intervals (e.g., approximately one-thousand unit intervals (or ˜1KUI)). The training supersequences may additionally be used for one ormore of deskew, configuration and for communicating initializationtarget, lane map, etc. The EIEOS can be used for one or more oftransitioning a lane from inactive to active state, screening for goodlanes, identifying symbol and TS boundaries, among other examples.

In one embodiment, the clock can be embedded in the data so there are noseparate clock lanes. The flits sent over the lanes can be scrambled tofacilitate clock recovery. The receiver clock recovery unit, as oneexample, can deliver sampling clocks to a receiver (i.e. the receiverrecovers clock from the data and uses it to sample the incoming data).Receivers in some implementations continuously adapt to an incoming bitstream. By embedding the clock, pinout can be potentially reduced.However, embedding the clock in the in-band data can alter the manner inwhich in-band reset is approached. In one embodiment, a blocking linkstate (BLS) can be utilized after initialization. Also, electricalordered set supersequences may be utilized during initialization tofacilitate the reset (e.g., as described above), among otherconsiderations. The embedded clock can be common between the devices ona link and the common operational clock can be set during calibrationand configuration of the link. For instance, HPI links can reference acommon clock with drift buffers. Such implementation can realize lowerlatency than elastic buffers used in non-common reference clocks, amongother potential advantages. Further, the reference clock distributionsegments may be matched to within specified limits.

In some implementations, HPI may support flits with a width that is, insome cases, not a multiple of the nominal lane width (e.g. using a flitwidth of 192 bits and 20 lanes as a purely illustrative example).Indeed, in implementations permitting partial width transmitting states,the number of lanes over which flits are transmitted can fluctuate, evenduring the life of the link. For example, in some instances, the flitwidth may be a multiple of the number of active lanes at one instant butnot be a multiple of the number of active lanes at another instant(e.g., as the link changes state and lane width). In instances where thenumber of lanes is not a multiple of a current lane width (e.g., theexample of a flit width of 192 bits on 20 lanes), in some embodiments,consecutive flits can be configured to be transmitted to overlap onlanes to thereby preserve bandwidth (e.g., transmitting five consecutive192 bit flits overlapped on the 20 lanes).

FIG. 8 illustrates a representation of transmission of consecutive flitsoverlapped on a number of lanes. For instance, FIG. 8 shows arepresentation of five overlapping 192-bit flits sent over a 20 lanelink (the lanes represented by columns 0-19). Each cell of FIG. 8represents a respective “nibble” or grouping of four bits (e.g., bits4n+3:4n) included in a flit sent over a 4UI span. For instance, a 192bit flit can be divided into 48 four-bit nibbles. In one example, nibble0 includes bits 0-3, nibble 1 includes bits 4-7, etc. The bits in thenibbles can be sent so as to overlap, or be interleaved (e.g.,“swizzled”), such that higher-priority fields of the flit are presentedearlier, error detection properties (e.g., CRC) are retained, amongother considerations. Indeed, a swizzling scheme can also provide thatsome nibbles (and their respective bits) are sent out of order (e.g., asin the examples of FIGS. 8 and 9). In some implementations, a swizzlingscheme can be dependent on the architecture of the link layer and formatof the flit used in the link layer.

The bits (or nibbles) of a flit with a length that is not a multiple ofthe active lanes can be swizzled, such as according to the example ofFIG. 8. For instance, during the first 4UI, nibbles 1, 3, 5, 7, 9, 12,14, 17, 19, 22, 24, 27, 29, 32, 34, 37, 39, 42, 44 and 47 can be sent.Nibbles 0, 2, 4, 6, 8, 11, 13, 16, 18, 21, 23, 26, 28, 31, 33, 36, 38,41, 43, and 46 can be sent during the next 4UI. In UIs 8-11, only eightnibbles remain of the first flit. These final nibbles (i.e., 10, 15, 20,25, 30, 40, 45) of the first flit can be sent concurrently with thefirst nibbles (i.e., nibbles 2, 4, 7, 9, 12, 16, 20, 25, 30, 35, 40, 45)of the second flit, such that the first and second flits overlap or areswizzled. Using such a technique, in the present example, five completeflits can be sent in 48UI, with each flit sent over a fractional 9.6 UIperiod.

In some instances, swizzling can result in periodic “clean” flitboundaries. For instance, in the example of FIG. 8, the starting 5-flitboundary (the top line of the first flit) may also be referred to as aclean flit boundary since all lanes are transmitting starting nibblefrom same flit. Agent link layer logic can be configured to identifyswizzling of lanes and can reconstruct the flit from the swizzled bits.Additionally, physical layer logic can include functionality foridentifying when and how to swizzle a stream of flit data based on thenumber of lanes being used at the moment. Indeed, in a transition fromone link width state to another, agents can configure themselves toidentify how swizzling of the data stream will be employed.

As noted above, links can transition between lane widths, in someinstances operating at an original, or full, width and latertransitioning to (and from) a partial width utilizing fewer lanes. Insome instances, the defined width of a flit may be divisible by thenumber of lanes. For instance, the example of FIG. 9 illustrates such anexample, where the 192-bit flit of the previous examples is transmittedover an 8-lane link. As represented in FIG. 9, 4-bit nibbles of a192-bit flit can be evenly distributed and transmitted over 8 lanes(i.e., as 192 is a multiple of 8). Indeed, a single flit may be sentover 24 UI when operating at an 8-lane partial width. Further, each flitboundary can be clean in the example of FIG. 9. While clean flitboundaries can simplify the state transitions, determinism, and otherfeatures, allowing for swizzling and occasional jagged flit boundariescan allow for the minimization of wasted bandwidth on a link.

Additionally, while the example of FIG. 9, shows lanes 0-7 as the lanesthat remained active in a partial width state, any set of 8 lanes canpotentially be used. Note also that the examples above are for purposesof illustration only. The flits can potentially be defined to have anywidth. Links can also have potentially any link width. Further, theswizzling scheme of a system can be flexibly constructed according tothe formats and fields of the flit, the preferred lane widths in asystem, among other considerations and examples.

The operation of the HPI PHY logical layer can be independent of theunderlying transmission media provided the latency does not result inlatency fixing errors or timeouts at the link layer, among otherconsiderations.

Link Layer

The Link layer can abstract the Physical layer from the Protocol layer,handle flow control between two protocol agents, and provide virtualchannel services to the Protocol layer (Message Classes) and Routinglayer (Virtual Networks). In some implementations, the Link layer candeal with a fixed quantum of information, termed a flit. In one example,the flit can be defined to be 192 bits in length. However, any range ofbits, such as 81-256 (or more) may be utilized in different variations.A large flit size, such as 192 bits, may include format, cyclicredundancy check (CRC), error correction code (ECC), and other features.For instance, a larger flit length can also permit the CRC field to beexpanded (e.g., to 16 bits) to handle the larger flit payload. Thenumber of phits or unit intervals (UI) (e.g., the time used to transfera single bit or phit, etc.) to transfer a single flit can vary with linkwidth. For instance, a 20 lane or bit link width can transfer a single192 bit flit in 9.6 UI, while an 8 lane link width transfers the sameflit in 24 UI, among other potential examples. The link layer creditingand protocol packetizing can also be based on a flit.

FIG. 10 illustrates a representation 1000 of a generalized flit for an 8lane link width. Each column of the representation 1000 can symbolize alink lane and each row a respective UI. In some implementations, asingle flit can be subdivided into two or more slots. Distinct messagesor link layer headers can be included in each slot, allowing multipledistinct, and in some cases, independent messages corresponding topotentially different transactions to be sent in a single flit. Further,the multiple messages included in slots of a single flit may also bedestined to different destination nodes, among other examples. Forinstance, the example of FIG. 10 illustrates a flit format with threeslots. The shaded portions can represent the portion of the flitincluded in a respective slot.

In the example of FIG. 10, three slots, Slots 0, 1, and 2, are provided.Slot 0 can be provided 72 bits of flit space, of which 22 bits arededicated to message header fields and 50 bits to message payload space.Slot 1 can be provided with 70 bits of flit space, of which 20 bits arededicated to message header fields and 50 bits to message payload space.The difference in message header field space between can be optimized toprovide that certain message types will be designated for inclusion inSlot 0 (e.g., where more message header encoding is utilized). A thirdslot, Slot 2, can be provided that occupies substantially less spacethan Slots 0 and 1, in this case utilizing 18 bits of flit space. Slot 2can be optimized to handle those messages, such as acknowledges, creditreturns, and the like that do no utilize larger message payloads.Additionally, a floating payload field can be provided that allows anadditional 11 bits to be alternatively applied to supplement the payloadfield of either Slot 0 or Slot 1.

Continuing with the specific example of FIG. 10, other fields can beglobal to a flit (i.e., apply across the flit and not to a particularslot). For instance, a header bit can be provided together with a 4-bitflit control field that can be used to designate such information as avirtual network of the flit, identify how the flit is to be encoded,among other examples. Additionally, error control functionality can beprovided, such as through a 16-bit cyclic CRC field, among otherpotential examples.

A flit format can be defined so as to optimize throughput of messages onthe Link layer. Some traditional protocols have utilized unslotted,smaller flits. For instance, in QPI an 80-bit flit was utilized. Whilethe flit throughput of a larger (e.g., 192-bit flit) may be lower,message or packet throughput can be increased by optimizing use of theflit data. For instance, in some implementations of QPI, the entire80-bit flit space was utilized regardless of the message size or type.By subdividing a larger flit into slots of predetermined lengths andfields, the 192 flit length can be optimized realizing higher efficiencyeven in instances when one or more of the available slots are sometimesunused. Indeed, Link layer traffic can be assumed to include manydifferent types of messages and traffic, including messages and packetswith varying header lengths and fields. The respective lengths andorganization of slots defined in a flit can be defined so as tocorrespond with the statistical or expected frequency of variousmessages and the needs of these messages. For instance, two larger slotscan be defined for every small slot, to accommodate an expectedstatistical frequency of messaging using these larger message types andheader lengths, among other example. Further, flexibility can also beprovided to further accommodate the varied traffic, such as through afloating payload field, as in the example of FIG. 10. In some instances,a flit format can be fixed, including the bits dedicated to particularslots in the flit.

In the example of FIG. 10, a “Hdr” field can be provided for the flitgenerally and represent a header indication for the flit. In someinstances, the Hdr field can indicate whether the flit is a header flitor a data flit. In data flits, the flit can still remain slotted, butomit or replace the use of certain fields with payload data. In somecases, data fields may include an opcode and payload data. In the caseof header flits, a variety of header fields can be provided. In theexample of FIG. 10, “Oc” fields can be provided for each slot, the Ocfield representing an opcode. Similarly, one or more slots can have acorresponding “msg” field representing a message type of thecorresponding packet to be included in the slot, provided the slot isdesigned to handle such packet types, etc. “DNID” fields can represent aDestination Node ID, a “TID” field can represent a transaction ortracker ID, a “RHTID” field can represent either a requestor node ID ora home tracker ID, among other potential fields. Further, one or moreslots can be provided with payload fields. Additionally, a CRC field canbe included within a flit to provide a CRC value for the flit, amongother examples.

In some implementations, link width can vary during the life of thelink. For instance, the Physical layer can transition between link widthstates, such as to and from a full or original lane width and adifferent or partial lane width. For example, in some implementations, alink can be initialized to transfer data over 20 lanes. Later, the linkcan transition to a partial width transmitting state where only 8 lanesare actively used, among many other potential examples. Such lane widthtransitions can be utilized, for instance, in connection with powermanagement tasks governed by one or more power control units (PCU) amongother examples.

As noted above, link width can influence flit throughput rate. FIG. 11is a representation of an example 192-bit flit sent over an 8 lane link,resulting in throughput of the flit at 24UI. Further, as shown in theexample of FIG. 11, bits of the flit can be sent out of order in someinstances, for example, to send more time-sensitive fields earlier inthe transfer (e.g., flit type fields (e.g., data or header flit),opcodes, etc.), preserve or facilitate particular error detection orother functionality embodied in the flit, among other examples. Forinstance, in the example of FIG. 11, bits 191, 167, 143, 119, 95, 71,47, and 23 are sent in parallel on lanes L7 through L0 during a first UI(i.e., UI0) of transfer, while bits 168, 144, 120, 96, 72, 48, 24, and 0are sent during the 24^(th) (or final) UI of the flit transfer (i.e.,UI23). It should be appreciated that other ordering schemes, flitlengths, lane widths, etc. can be utilized in other implementations andexamples.

In some instances, the length of the flit can be a multiple of thenumber of active lanes. In such instances, the flit can be transmittedevenly on all active lanes and transfer of the flit can endsubstantially simultaneously at a clean (i.e., non-overlapping)boundary. For example, as shown in the representation of FIG. 8, bits ofa flit can be considered to be transmitted in consecutive groupings of 4bits, or “nibbles.” In this example, a 192 bit flit is to be transferredover an 8 lane link. As 192 is a multiple of 8, the entire flit can becleanly transferred over the 8 lane link in 24 UI. In other instances,the flit width may not be a multiple of the number of active lanes. Forinstance, FIG. 9 shows another representation of an example 192 bittransferred over 20 lanes. As 192 is not evenly divisible by 20,transfer of the full flit would require a non-integer number ofintervals (e.g., 9.6 UI). In such cases, rather than wasting “extra”lanes not utilized during the 10th UI of transfer, a second overlappingflit can be transferred with the final bits of a preceding flit. Suchoverlapping, or swizzling, of the flits can result in jagged flitboundaries and flit bits sent out of order in some implementations. Thepattern utilized for the transfer can be configured to allow moretime-sensitive fields of the flit to be transferred earlier in the flit,preservation of error detection and correction, among otherconsiderations. Logic can be provided in one or both of the Physical andLink layers to transfer flit bits according to such patterns anddynamically change between patterns based on the current link width.Further logic can be provided to re-order and re-construct flits fromsuch swizzled or ordered bit streams, among other examples.

In some implementations, flits can be characterized as header flits(e.g., bearing packet header data) or data flits (e.g., bearing packetpayload data). Returning to FIG. 10, a flit format can be defined thatincludes three (3) distinct slots (e.g., 0, 1, and 2), allowing up tothree headers to be transferred in a single flit (e.g., one header ineach slot). In the example of FIG. 10, three slots, Slots 0, 1, and 2,are provided. Slot 0 can be provided 72 bits of flit space, of which 22bits are dedicated to message header fields and 50 bits to messagepayload space. Slot 1 can be provided with 70 bits of flit space, ofwhich 20 bits are dedicated to message header fields and 50 bits tomessage payload space. The difference in message header field spacebetween can be optimized to provide that certain message types will bedesignated for inclusion in Slot 0 (e.g., where more message headerencoding is utilized). A third slot, Slot 2, can be provided thatoccupies substantially less space than Slots 0 and 1, in this caseutilizing 18 bits of flit space. Slot 2 can be optimized to handle thosemessages, such as acknowledges, credit returns, and the like that do noutilize larger message payloads. Additionally, a floating payload fieldcan be provided that allows an additional 11 bits to be alternativelyapplied to supplement the payload field of either Slot 0 or Slot 1.

In some implementations, by allowing a field to float between the twoslots, extra bits can be provided as needed for certain messages whilestill staying within a predefined flit length (e.g., 192 bits) andmaximizing the utilization of the bandwidth. Turning to the examples ofFIG. 19, two instances 1905, 1910 of an example 192-bit flit are shownon an 8 lane data link. In one instance, a flit (e.g., 1905) can includethree slots, Slots 0, 1, and 2. Each of Slots 0 and 1 can include 50-bitpayload fields. The floating field can be provided to alternativelyextend the payload field of the either Slot 0 or Slot 1 by the fieldlength (e.g., 11 bits) of the floating field. The use of a floatingfield can further extend the efficiency gains provided through adefined, multi-slot flit format. The sizing of the slots within theflit, and the types of messages that can be placed in each slot, canpotentially provide increased bandwidth even with a reduced flit rate.

In the particular example of FIG. 10, the messages that can use Slots 1and 2 can be optimized, reducing the number of bits to be set aside toencode these slots' opcodes. When a header having more bits that Slot 0can provide enters the Link layer, slotting algorithms can be providedto allow it to take over Slot 1 payload bits for additional space.Special control (e.g. LLCTRL) flits may also be provided that consumeall three slots worth of bits for their needs. Slotting algorithms mayalso exist to allow individual slots to be utilized while other slotscarry no information, for cases where the link is partially busy.

In one embodiment, the Link Layer can additionally define specialcontrol flits that may be used, for instance, for debug messages andother uses. In one example, LLCTRL-DEBUG flits may be sent by the LinkLayer when an enable debug control field is set. When this bit is notset, LLCTRL-DEBUG flits may not be transmitted on the link. DebugPackets may be important to expose internal states of devices connectedby HPI that are otherwise inaccessible. The contents of debug packetscan also be implementation specific. Contents could include things likebranch info (source and target IPs), time-stamps, indication of aninternal event trigger, and so on. The exposed data can be, forinstance, by monitoring devices such as logic analyzers forpost-processing and failure analysis. An example flit encoding of aDebug message type is illustrated in FIG. 12.

In some implementations, principles and protocols of a general purposeI/O interconnect (GPIO) architecture, such as HPI, can be utilized toimplement a buffered memory interface and corresponding protocol. Forinstance, the physical and link layer definitions outlined above canalso be implemented in a buffered memory protocol. Indeed, logic used tosupport the physical and link layers of the GPIO protocol can be reusedat interfaces supporting the buffered memory protocol. The bufferedmemory protocol can also share message classes, such as a request,response, and writeback message class, among other examples. Whileopcode values within the buffered memory protocol message can beinterpreted differently than in HPI (or another GPIO protocol), the samegeneral packet formats can be utilized in both the buffered memoryprotocol and the GPIO interconnect upon which it is built. Indeed,additional packet and flit fields can be unique to both the bufferedmemory protocol and GPIO interconnect protocol.

In some implementations, a buffered memory protocol utilizing thephysical and link layers of HPI can be a transactional interface andprotocol. This can permit data to be returned out of order to a seriesof received requests (e.g., read requests). The buffered memoryinterface can be used to interconnect a buffer chip to a CPU in someimplementations. In one implementation, While some protocols send DRAMcommands such as Activate, Read, Write, and Refresh across a bufferedmemory bus, in some implementations of the buffered memory protocoldiscussed herein, a read or write command can simply be sent with anaddress. The buffer chip can then decode the specific rank and bank, andbreaks the request down to DRAM commands. Speculative and demand readscan also be supported.

Turning to FIG. 13, a simplified block diagram 1300 is shownillustrating an example topology of a computing system including CPUdevices 1305, 1310 interconnected by an HPI-compliant link. Each CPU1305, 1310 can be likewise connected to one or more respective bufferdevices 1315 a-1 using corresponding buffered memory protocol links(“MemLink”). As noted above, in some implementations, the bufferedmemory protocol interconnect can be based on the GPIO protocol, in thatthe physical and link layers of the buffered memory protocols are basedon the same physical and link layer definitions of the GPIO protocol(e.g., HPI). Although not illustrated in FIG. 13, the CPUs 1305, 1310can be further connected to one or more downstream devices using theGPIO protocol.

As further shown in the example of FIG. 13, buffer devices 1315 a-1 canbe connected to memory devices, such as dual in-line memory module(DIMM) devices. The memory corresponding to each buffer device can beconsidered local to the CPU (e.g., 1305, 1301) to which the bufferdevice is connected. However, other devices (including the other CPU)can access the memory by other sockets using GPIO protocol-compliantlinks. In some implementations, a port running the buffered memoryprotocol may only support the commands for communicating with the memoryand only support the buffered memory protocol (i.e., not the GPIOprotocol and the buffered memory protocol). Additionally, in someimplementations, the GPIO interconnect protocol may support routing andindicate such information (e.g., in its packets) such as the requestingand destination node identifiers. The buffered memory protocol, on theother hand, may be a point-to-point interface that does not utilizerouting. Consequently, some fields used in the GPIO protocol may bedispensed with in packets sent using the buffered memory interfaces.Instead, fields can be designated for use in carrying address decodeinformation host to buffer, among other examples.

In further implementations, buffer devices 1315 a-1 can support a twolevel memory topology with some amount of fast memory (e.g., DRAM)serving as a cache for a larger, slower memory (e.g., non-volatilememory). In one such implementation, one or more of the buffer devices1315 a-1 can use DDR as near, fast memory and transactional DDR DIMMs asthe larger “far” memory, among other examples. Transactional DIMMs canutilize protocols (e.g., DDR-Transactional (DDR-T)) to communicate to avolatile memory single in-line memory module (SIMM) using transactionalcommands.

Turning to FIG. 14, a simplified block diagram 1400 is presented showinga a more detailed representation of a buffer chip 1315 connected to nearand far memory modules (e.g., 1405, 1410). As noted in the exampleabove, the buffer chip 1315 can be communicatively coupled to a CPUdevice 1305 through a buffered memory protocol interconnect link 1415utilizing a defined buffered memory protocol. The buffer chip 1315 caninclude logic to perform additional functions. For instance, animplementation of the buffer chip 1315 can include a memory controller1420, a scheduler 1425, flow control logic 1430, and error detectionlogic 1435. Logic can be implemented using hardware circuitry, firmware,and/or software.

In some implementations, the memory controller 1420 can include logic toconvert requests into the memory specific protocol (e.g., DDR4) of itsmemory devices. The memory controller 1420 can abstract the details ofthese memory-specific protocols from the host controller logic 1440 ofthe CPU 1305 (or another host) which communicates with the buffer chip1315 using link 1415. Host controller logic 1440 can translate CPU (orhost) addresses to memory addresses, including permuting across channelsand sockets, among other functionality. Scheduler 1425 can include logicto reorder and arbitrate responses to requests in order to achievehigher performance. Buffer chip 1315 can additionally offer featuressuch as flow control and error detection. In some implementations, flowcontrol logic 1430 can be embodied in scheduler 1425 or otherwiseinteroperate with scheduler 1425 to ensure more efficiency and higherperformance of the buffer chip 1315. Error detection logic 1440 caninclude logic to support an error correction code detection scheme aswell as logic for initiating correction or handling of detected errors.

As noted above, the link layer of a buffered memory protocol can bebased on that of a GPIO protocol. For instance, the 192-bit, 3-slot flitof an HPI protocol (e.g., described and shown in FIG. 10) can beutilized in a buffered memory protocol. A packet in the buffered memoryprotocol can embody an entire command. A packet can be implemented asone or more flits. FIGS. 15A-15C can represent implementations of a flitin a buffered memory protocol. In some implementations, read data can besent back from the buffer device to a host via a combination of headeredand headerless memory data (MemData) packets. An entire cache line canbe transferred in three headerless flits (rather than five, as withheadered flits). A headerless packet can thereby provide for higherthroughput on what may be the most constrained portion of the interface.In order to fit into three flits, the headerless packet removes a numberof fields and makes assumptions as to their values. The assumptions mademay be true for most, but not all read returns, If any assumptions arenot true, the headered packet is to be used, which contains all fields.In some implementations, a headered packet can begin a sequence of readreturns to permit early identification of a tracker identifier (e.g.,HTID, RTID, etc.) for use in mapping a read return to a read request ina transactional buffered memory protocol.

In some implementations, headerless packets can contain the informationoutlined in Table 1:

TABLE 1 Headerless Packet Fields Field Qty Description Data 512 64 bytecache line data Directory 2 Two directory bits, which are stored withthe data, generally as part of the ECC bits HTID 11 Request transactionID. Due to early HTID, the packet contains the HTID for the next readdata packet CRC 48 16 bits per of CRC per flit HIB 3 Header indicationbit. One per flit. This bit is always 0 for each flit of headerlesspackets.Further, FIGS. 15A-15C illustrate a sequence of three flits (e.g., flit0 (at FIG. 15A), flit 1 (at FIG. 15B), and flit 2 (at FIG. 15C)) thatcan be used to transmit an entire cache line in a read return. Table 2provides a summary of an example of the three flits utilized in aheaderless packet:

TABLE 2 Headerless Packet Flit Contents Data flit 0 Data from the first32 bytes of the cache line. Data flit 1 Remainder of the first 32 bytes,plus the first part of the second 32 bytes. Data flit 2 Remainder of thesecond 32 bytes

An HIB bit can be set to indicate whether the packet is headered orheaderless. HIB may be set to indicate a headerless packet only for dataflits following a header flit. This allows certain other packets to beinterleaved in the middle of a packet. When the interleaved packet iscompleted, the data portions of the original headered packet can be sentwith HIB=0 in the flits. This technique can also be used to delay thedata packets by inserting null flits. In some implementations, the startof a headerless packet is signaled by the HIB bit being a 0 whennormally a header would be required. All other packets, including nullflits, may contain a header with the HIB bit set.

As introduced above, headered packets may be a total of five flits inlength, and may include one header flit and 4 data flits. In order toreduce idle latency, the headered flit may send data flit 1 twice. Thefirst time it is sent with only the data from the first 32 bytes and isreferred to as flit 1 a. This allows the flit to be sent before thesecond 32 bytes of data is available from the memory devices. The resultis that the critical 32 bytes chunk of data arrives earlier at the hostfor the idle case. Flit 1 can then be sent a second time, this timecomplete. It will contain a repeat of the first 32 byte data portionplus the second 32 byte portion. Table 3 describes the five flits of aheadered packet:

TABLE 3 Headered Packet Flit Contents Header Packet Header Data flit 0Data from the first 32 bytes of the cache line Data flit 1a Remainder ofthe first 32 bytes only. Second 32-byte portion is reserved. The otherflit payload bits (RTID[6:3]) are identical to Data Flit 1. Data flit 1Complete Flit 1. First 32-byte portion is resent. Second 32-byte portionalso included. Data flit 2 Remainder of the second 32 bytes.In some implementations, headered MemData packets may not be cancelledand are to be followed by the data flits. A header is not to be sentuntil it known that the data can also be sent. The full cache line maynot be accumulated in the buffer before the header is sent.

Systems can be particularly sensitive to latency. Memory latency can becritical for CPU performance. In implementations utilizing transactionalbuffered memory interfaces, additional CPU latency can be dedicated toidentifying a transaction's tracker identifier (ID) (such as a HTID) ina buffer device's transaction response and matching the response to acorresponding transaction request. In some implementations, latency canbe reduced by having the buffer device pre-send a tracker identifier ofan upcoming response, such as a read return, to allow the CPU topre-process the tracker identifier and anticipate the arrival of thecorresponding response. Further, rather than interrupting presentresponse transmissions, the tracker ID of the upcoming response can bepre-sent by embedding the tracker ID in a current response. Indeed,interrupting data transmission for an earlier read in order to send anearly read identifier packet would decrease the bandwidth of the link.By embedding the tracker ID for an upcoming read in the data for readthat is already being transmitted, can help implement a high bandwidth,low latency data bus.

Turning to FIG. 16, a representation 1600 of an example communicationbetween a buffer device 1315 and CPU 1305 are illustrated. A number ofread requests (e.g., Requests 1, 2, and 3) can be sent from the CPU 1305to the buffer device 1315 to obtain data from memory devices connectedto the buffer device 1315. The buffer device 1315 and CPU 1305 can betransaction aware, allowing the buffer device 1315 to potentially returnread responses (e.g., Responses 1, 2, and 3) out-of-order, with theresponse ordering based on algorithms for improving performance of thebuffered memory interface, rather than a purely deterministic protocol.Each read request can include a tracker identifier and the buffer cantag responses to so that the CPU can determine which responsescorrespond to which requests. For instance, in the example of FIG. 16,read request 2 is sent to the buffer device 1315 immediately after readrequest 1, however, buffer device 1315 returns the response (i.e., readresponse 2) to read request 2, before returning the response (i.e., readresponse 1) to read request 1. The CPU can identify which responsespertain to which requests based on a tracker identifier included in theresponses. In some implementations, a tracker identifier can beimplemented in a field (e.g., an HTID, RHTID, RTID field, etc.) of aflit used to transmit the data to the CPU.

Turning to FIG. 17, an example of early transaction identification isillustrated. In this example, the CPU 1305 again sends a series of athree read requests (Read requests 1, 2, and 3). In some instance, thethree read requests can relate to entirely distinct transactions. Inother cases, two or more the read requests may be related (e.g., relatedto a common transaction between the CPU and another host, etc.). In theexample of FIG. 17, the buffer device 1315 can again determine that itis best to return responses to the requests out-of-order, with theresponse to request 2 being returned ahead of the response to request 1.The buffer device 1315, in this case, cannot send an early tracker IDfor read response 2 as no communications from the buffer device 1315 tothe CPU 1305 over a buffered memory link immediately preceded readresponse 2. However, in this instance, the buffer device 1315 canidentify that read response 1 is to follow read response 2 prior tosending read response 2. Additionally, the buffer device 1315 can notonly embed the tracker ID for read request 2 in read response 2, but thebuffer device 1315 can further encode another field (e.g., of a memorydata flit) of read response 2 to provide an early identification of thetracker ID for the response (i.e., read response 1) to read request 1 tothereby allow the CPU to perform pre-processing steps in advance of thearrival of the read response 1. Further, in this example, the bufferdevice 1315 can also identify, prior to sending read response 2, thatread response 3 is to be sent next after read response 2. Accordingly,buffer device 1315 can embed an early identification of the tracker IDfor read response 3 within read response 2 to allow CPU to performpre-processing steps in advance of the arrival of read response 3. Forinstance, the CPU might look up the routing information necessary tosend the data to the requester in advance of the arrival of the readdata.

Sending an early tracker ID can allow the host controller of a receivingCPU (or other host) to get advanced knowledge of which read is beingreturned. In traditional memory systems utilizing deterministicinterface, there is advanced knowledge of when read data will bereturned. The host, in such system, is thus able to use this advancedknowledge to reduce latency. As a transaction buffered memory interfaceallows the read data to be returned out of order (e.g., by providing thescheduler within the buffer device), the risk exists that the hostcontroller will lose this advanced knowledge.

Continuing with the example of FIG. 17, after read response 3 is sentfrom the buffer device 1315 to the CPU 1305, a read request 4 isreceived from the CPU 1305 at the buffer device 1315. Further, no otherrequests may be received from the CPU 1305 that compete for schedulingby the buffer device 1315, allowing the buffer device to determine thatit can send a response (i.e., Read response 4) to read request 4 as itsnext response. The buffer device 1315 can further determine that noopportunities exist to send an early tracker ID for read response 4 asno other responses exist in which the response can be embedded. In otherimplementations, the buffer device 1315 may determine that bandwidthexists on the buffered memory link connecting the buffer device 1315 tothe CPU 1305 to allow the buffer device to send a control flit or otherflit to the CPU 1305 to pre-identify the tracker ID of read response 4in advance of the sending of read response 4 (and, in some cases, whilethe buffer device 1315 prepares read response 4), among otherimplementations. Further, as no other responses are identified aspending by the buffer device 1315 as it prepare read response 4 forsending, no early tracker ID may be sent with the read response 4.

In one example implementation, headerless MemData packets and the dataportion of headered MemData packets can be encoded to contain a trackerID of the next headerless packet. For example, a sequence of reads canbe begin with a headered packet. This first headered packet can includeits own tracker ID. In one implementation, the first headered packet'stracker ID can be encoded in its corresponding header flit. A sequenceof other packets can follow this first packet and data flits of thisfirst packet can be encoded with the tracker ID of the next packetfollowing the first packet in the sequence. In some instances, otherpackets may be inserted between the headerless packets without breakingthe chain of tracker IDs, with only another headered MemData packetcapable of breaking the chain (e.g., by indicating a new first packettracker ID). If the buffer device does not know the tracker ID for thenext headerless data packet (e.g., the packet has not yet been receivedor processed by the buffer device), the buffer device can insert adummy, null, or other equivalent value as the tracker ID of the nextpacket. This can trigger the next memory read packet being sent by thebuffer device as a first headered packet in a next sequence.Additionally, if after sending a tracker ID in a headerless packet, thebuffer device determines that it cannot sent that specific data, it cansimply send a headered packet as the next MemData packet. Reception ofthe tracker ID by the host device (e.g., CPU 1305) from the bufferdevice (e.g., 1315) may not guarantee that this data will arrive next(e.g., as it can be pre-empted by a headered MemData packet), amongother examples and implementations.

Table 4 illustrates a sequence of flits corresponding to at least 5transactions (i.e., transactions A, B, C, X, Y) and further illustrateshow a tracker ID field may be utilized to send an early identificationof an upcoming read return's tracker ID to the host:

TABLE 4 Early Tracker IDs in a Sequence of Flits Flit Command TID Notes0 Header A A MemData Header with HTID starts sequence 1 Data0 A B Datafor A. This data technically goes with the Header A packet, but it isexactly the same format as the headerless packet 2 Data1a A — 2^(nd)data flit for A, first half cache line portion only 3 Data1 A — 2^(nd)data flit for A, complete 4 Data2 A — 3^(rd) data flit for A 5 Data0 B CData for B, sent as a headerless packet. HTID is for data C. 6 Data1 B —2^(nd) data flit for B 7 Data2 B — 3^(rd) data flit for B 8 WriteCompletion — Or any other non-read command. Other commands do not cancelthe early HTID chain. 9 Data0 C Dummy No further read data will beavailable in the TMXB, so a Dummy HTID is sent. 10 Data1 C — 2^(nd) dataflit for C 11 Data2 C — 3^(rd) data flit for C 12-49 Multiple flits (no— read data) 50 Header X X New headered packet, with HTID of X 51 Data0X Y Data for X, sent as a headerless packet. HTID is for data Y. 52Data1a X — 2^(nd) data flit for X, first half cache line portion only 53Data1 X — 2^(nd) data flit for X, complete 54 Data2 X — 3^(rd) data flitfor X 55 Data0 Y Dummy No further read data will be available in theTMXB, so a Dummy HTID is sent. 56 Data1 Y — 2^(nd) data flit for Y 57Data2 Y — 3^(rd) data flit for Y 58-on Multiple flits (no — read data)In the example of TABLE 4, the tracker ID (TID) is shown in the firstflit of the data (however, this can actually be actually spread acrossall three or four flits of the same packet). For instance, a header flit(Header A) of the read return for transaction A includes the TID fortransaction A. As the read return for transaction B is to immediatelyfollow the read return for transaction A, an opportunity to send anearly TID for transaction B can be identified. Accordingly, data flitData0 A of transaction A can be encoded with the TID for transaction B,as shown in the example of TABLE 4. Likewise, a data flit of transactionB's read return (e.g., flit Data0 B) can be encoded to provide an earlyindication of the TID for the upcoming read return for transaction C.

Continuing with the example of TABLE 4, prior to sending the read returnfor transaction C, the buffer device can determine that no opportunitiesexist for sending an early TID with the read return of transaction C.Indeed, flits 12-49 include no read data. Accordingly, instead ofsending an early TID with the read return of transaction C, dummy datacan be encoded in a field of the data flit (e.g., flit Data0 C) toindicate that the field in which early TIDs are encoded is to beignored.

As noted above, non-read operations can interrupt a series of readreturns without interfering with an early TID notification for asubsequent read return. For instance, as shown in the example of TABLE4, flit 5 (Data0 B) is encoded with the early TID for read return C butan intervening Write Completion flit (flit 8) is sent between the readreturns for transaction B and C. The write completion flit, however,does not interfere with the CPU's ability to map the next read return(e.g., read return C) that corresponds to the last sent early TID (e.g.,in flit Data0 B), among other features. As another example, if an errorcondition occurs such that the data cannot be sent for the TID that hasalready been sent, the transfer can be cancelled (e.g., implicitly orexplicitly). When a headered MemData packet is sent, the previous TIDcan be cancelled (e.g., to account for an error, such as an ECC error,etc.), among other examples.

Alternative mechanisms can be employed to augment or substitute featuresof an early tracker ID identification solution, such as presented above.For instance, dedicated wiring (e.g., sideband wiring) can be providedbetween a buffer device (e.g., 1315) and a corresponding host (e.g., CPU1305) on which the tracker IDs of upcoming read responses can be sent.Such sideband channels may be unnecessary, however, in implementationswhere the early tracker ID identification is sent in-band, embedded witha read return of another read request.

In one example of a transaction buffered memory interface, a memorycontroller of a buffer device (e.g., 1315) can know when a read isscheduled exactly when data will return from corresponding memory. As aresult, the buffer device can optimize performance by attempting tominimize read latency by preparing for the arrival of the read data,including sending an indication to the system agent of the CPU beforethe read data has arrived in order to allow early assembly of the headerfor the read data. This can minimize the latency because the read datacan be forwarded as soon as it arrives. This, coupled with functionalityfor sending early identification of tracker IDs of the responses to thebuffer device's host, can improve the efficiency and performance of thesystem by reducing latency, among other example advantages.

HPI and an associated buffered memory access protocol can beincorporated in any variety of computing devices and systems, includingmainframes, server systems, personal computers, mobile computers (suchas tablets, smartphones, personal digital systems, etc.), smartappliances, gaming or entertainment consoles and set top boxes, amongother examples. For instance, referring to FIG. 18, shown is a blockdiagram of a second system 1800 in accordance with an embodiment of thepresent invention. As shown in FIG. 18, multiprocessor system 1800 is apoint-to-point interconnect system, and includes a first processor 1870and a second processor 1880 coupled via a point-to-point interconnect1850. Each of processors 1870 and 1880 may be some version of aprocessor. In one embodiment, 1852 and 1854 are part of a serial,point-to-point coherent interconnect fabric, such as a high-performancearchitecture. As a result, the invention may be implemented within theQPI architecture.

While shown with only two processors 1870, 1880, it is to be understoodthat the scope of the present invention is not so limited. In otherembodiments, one or more additional processors may be present in a givenprocessor. Processors 1870 and 1880 are shown including integratedmemory controller units 1872 and 1882, respectively. Processor 1870 alsoincludes as part of its bus controller units point-to-point (P-P)interfaces 1876 and 1878; similarly, second processor 1880 includes P-Pinterfaces 1886 and 1888. Processors 1870, 1880 may exchange informationvia a point-to-point (P-P) interface 1850 using P-P interface circuits1878, 1888. As shown in FIG. 18, IMCs 1872 and 1882 couple theprocessors to respective memories, namely a memory 1832 and a memory1834, which may be portions of main memory locally attached to therespective processors.

Processors 1870, 1880 each exchange information with a chipset 1890 viaindividual P-P interfaces 1852, 1854 using point to point interfacecircuits 1876, 1894, 1886, 1898. Chipset 1890 also exchanges informationwith a high-performance graphics circuit 1838 via an interface circuit1892 along a high-performance graphics interconnect 1839.

A shared cache (not shown) may be included in either processor oroutside of both processors; yet connected with the processors via P-Pinterconnect, such that either or both processors' local cacheinformation may be stored in the shared cache if a processor is placedinto a low power mode.

Chipset 1890 may be coupled to a first bus 1816 via an interface 1896.In one embodiment, first bus 1816 may be a Peripheral ComponentInterconnect (PCI) bus, or a bus such as a PCI Express bus or anotherthird generation I/O interconnect bus, although the scope of the presentinvention is not so limited.

As shown in FIG. 18, various I/O devices 1814 are coupled to first bus1816, along with a bus bridge 1818 which couples first bus 1816 to asecond bus 1820. In one embodiment, second bus 1820 includes a low pincount (LPC) bus. Various devices are coupled to second bus 1820including, for example, a keyboard and/or mouse 1822, communicationdevices 1827 and a storage unit 1828 such as a disk drive or other massstorage device which often includes instructions/code and data 1830, inone embodiment. Further, an audio I/O 1824 is shown coupled to secondbus 1820. Note that other architectures are possible, where the includedcomponents and interconnect architectures vary. For example, instead ofthe point-to-point architecture of FIG. 18, a system may implement amulti-drop bus or other such architecture.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

A design may go through various stages, from creation to simulation tofabrication. Data representing a design may represent the design in anumber of manners. First, as is useful in simulations, the hardware maybe represented using a hardware description language or anotherfunctional description language. Additionally, a circuit level modelwith logic and/or transistor gates may be produced at some stages of thedesign process. Furthermore, most designs, at some stage, reach a levelof data representing the physical placement of various devices in thehardware model. In the case where conventional semiconductor fabricationtechniques are used, the data representing the hardware model may be thedata specifying the presence or absence of various features on differentmask layers for masks used to produce the integrated circuit. In anyrepresentation of the design, the data may be stored in any form of amachine readable medium. A memory or a magnetic or optical storage suchas a disc may be the machine readable medium to store informationtransmitted via optical or electrical wave modulated or otherwisegenerated to transmit such information. When an electrical carrier waveindicating or carrying the code or design is transmitted, to the extentthat copying, buffering, or re-transmission of the electrical signal isperformed, a new copy is made. Thus, a communication provider or anetwork provider may store on a tangible, machine-readable medium, atleast temporarily, an article, such as information encoded into acarrier wave, embodying techniques of embodiments of the presentinvention.

A module as used herein refers to any combination of hardware, software,and/or firmware. As an example, a module includes hardware, such as amicro-controller, associated with a non-transitory medium to store codeadapted to be executed by the micro-controller. Therefore, reference toa module, in one embodiment, refers to the hardware, which isspecifically configured to recognize and/or execute the code to be heldon a non-transitory medium. Furthermore, in another embodiment, use of amodule refers to the non-transitory medium including the code, which isspecifically adapted to be executed by the microcontroller to performpredetermined operations. And as can be inferred, in yet anotherembodiment, the term module (in this example) may refer to thecombination of the microcontroller and the non-transitory medium. Oftenmodule boundaries that are illustrated as separate commonly vary andpotentially overlap. For example, a first and a second module may sharehardware, software, firmware, or a combination thereof, whilepotentially retaining some independent hardware, software, or firmware.In one embodiment, use of the term logic includes hardware, such astransistors, registers, or other hardware, such as programmable logicdevices.

Use of the phrase ‘configured to,’ in one embodiment, refers toarranging, putting together, manufacturing, offering to sell, importingand/or designing an apparatus, hardware, logic, or element to perform adesignated or determined task. In this example, an apparatus or elementthereof that is not operating is still ‘configured to’ perform adesignated task if it is designed, coupled, and/or interconnected toperform said designated task. As a purely illustrative example, a logicgate may provide a 0 or a 1 during operation. But a logic gate‘configured to’ provide an enable signal to a clock does not includeevery potential logic gate that may provide a 1 or 0. Instead, the logicgate is one coupled in some manner that during operation the 1 or 0output is to enable the clock. Note once again that use of the term‘configured to’ does not require operation, but instead focus on thelatent state of an apparatus, hardware, and/or element, where in thelatent state the apparatus, hardware, and/or element is designed toperform a particular task when the apparatus, hardware, and/or elementis operating.

Furthermore, use of the phrases ‘to,’ ‘capable of/to,’ and or ‘operableto,’ in one embodiment, refers to some apparatus, logic, hardware,and/or element designed in such a way to enable use of the apparatus,logic, hardware, and/or element in a specified manner. Note as abovethat use of to, capable to, or operable to, in one embodiment, refers tothe latent state of an apparatus, logic, hardware, and/or element, wherethe apparatus, logic, hardware, and/or element is not operating but isdesigned in such a manner to enable use of an apparatus in a specifiedmanner.

A value, as used herein, includes any known representation of a number,a state, a logical state, or a binary logical state. Often, the use oflogic levels, logic values, or logical values is also referred to as 1'sand 0's, which simply represents binary logic states. For example, a 1refers to a high logic level and 0 refers to a low logic level. In oneembodiment, a storage cell, such as a transistor or flash cell, may becapable of holding a single logical value or multiple logical values.However, other representations of values in computer systems have beenused. For example the decimal number ten may also be represented as abinary value of 1010 and a hexadecimal letter A. Therefore, a valueincludes any representation of information capable of being held in acomputer system.

Moreover, states may be represented by values or portions of values. Asan example, a first value, such as a logical one, may represent adefault or initial state, while a second value, such as a logical zero,may represent a non-default state. In addition, the terms reset and set,in one embodiment, refer to a default and an updated value or state,respectively. For example, a default value potentially includes a highlogical value, i.e. reset, while an updated value potentially includes alow logical value, i.e. set. Note that any combination of values may beutilized to represent any number of states.

The embodiments of methods, hardware, software, firmware or code setforth above may be implemented via instructions or code stored on amachine-accessible, machine readable, computer accessible, or computerreadable medium which are executable by a processing element. Anon-transitory machine-accessible/readable medium includes any mechanismthat provides (i.e., stores and/or transmits) information in a formreadable by a machine, such as a computer or electronic system. Forexample, a non-transitory machine-accessible medium includesrandom-access memory (RAM), such as static RAM (SRAM) or dynamic RAM(DRAM); ROM; magnetic or optical storage medium; flash memory devices;electrical storage devices; optical storage devices; acoustical storagedevices; other form of storage devices for holding information receivedfrom transitory (propagated) signals (e.g., carrier waves, infraredsignals, digital signals); etc., which are to be distinguished from thenon-transitory mediums that may receive information there from.

Instructions used to program logic to perform embodiments of theinvention may be stored within a memory in the system, such as DRAM,cache, flash memory, or other storage. Furthermore, the instructions canbe distributed via a network or by way of other computer readable media.Thus a machine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer), but is not limited to, floppy diskettes, optical disks,Compact Disc, Read-Only Memory (CD-ROMs), and magneto-optical disks,Read-Only Memory (ROMs), Random Access Memory (RAM), ErasableProgrammable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), magnetic or optical cards, flashmemory, or a tangible, machine-readable storage used in the transmissionof information over the Internet via electrical, optical, acoustical orother forms of propagated signals (e.g., carrier waves, infraredsignals, digital signals, etc.). Accordingly, the computer-readablemedium includes any type of tangible machine-readable medium suitablefor storing or transmitting electronic instructions or information in aform readable by a machine (e.g., a computer).

The following examples pertain to embodiments in accordance with thisSpecification. One or more embodiments may provide an apparatus, asystem, a machine readable storage, a machine readable medium, hardware-and/or software-based logic, and a method to identify a sequence of readreturns to be sent to a host device over a transactional buffered memoryinterface, where the sequence includes at least a first read return to afirst read request and a second read return to a second read request. Atracker identifier of the second read return can be encoded in the firstread return and the first read return can be sent with the trackeridentifier of the second read return to the host device. The second readreturn can be sent to the host device after the first read return issent.

In at least one example, the interface is to include a link compliantwith a buffered memory link protocol.

In at least one example, the buffered memory link protocol incorporatesa physical layer and link layer of a general purpose input/output (GPIO)interconnect protocol.

In at least one example, the memory buffer device is to receive asequence of read requests from the host device and the sequence of readrequests correspond to the sequence of read returns.

In at least one example, a final one of the read returns in the sequenceis to be encoded to indicate a dummy value for an early transactionidentifier.

In at least one example, the first read return is to further include atracker identifier of the first read return.

In at least one example, the first read return is to include a headeredpacket and the second read return is to include a headerless packet.

In at least one example, the first read return is to include theheadered packet based on the first read return being the first readreturn in the sequence and the subsequent read returns in the sequenceare include headerless packets.

In at least one example, headered packets each include a set of flits toinclude one header flit and four data flits and headerless packets eachinclude a set of three data flits.

In at least one example, the tracker identifier of the second readreturn is to be encoded in a first data flit of the first read returnand the tracker identifier of the first read return is to be encoded inthe header flit of the first read return.

In at least one example, the first read request and first read returncorrespond to a first transaction and the second read request and secondread return correspond to a second transaction.

In at least one example, the sequence further includes a third readreturn to correspond to a third read request and the second read returnis to be encoded with a tracker identifier of the third read return.

One or more embodiments may provide an apparatus, a system, a machinereadable storage, a machine readable medium, hardware- and/orsoftware-based logic, and a method to send a first read request of afirst transaction to a buffer device, send a second read request of asecond transaction to the buffer device, and receive a first read returnfrom the buffer device. The first read return is to include a firsttracker identifier to indicate that the first read return corresponds tothe first read request, and is to further include an early secondtracker identifier to indicate that a next read return is to correspondto the second read request.

In at least one example, the receiver is further to receive the nextread return from the buffer device.

In at least one example, the host device is to further include a hostcontroller to map the first read return to the first read request basedon the first tracker identifier and map the second read return to thesecond read request.

In at least one example, the host device includes a central processingunit.

One or more embodiments may provide a system to include a first hostdevice, a buffer device connected to the first host device using abuffered memory link compliant with a memory access link protocol, oneor more memory devices connected to the buffer device, and a second hostdevice connected to the second host device using a particular linkcompliant with a general purpose input/output (GPIO) link protocol. Thebuffer device can identify a sequence of read returns to be sent to thefirst host device over the buffered memory link, encode a trackeridentifier of the second read return in the first read return, send thefirst read return with the tracker identifier of the second read returnto the first host device, and send the second read return to the firsthost device after the first read return is sent.

In at least one example, the host device is further to forward data ofat least one of the first and second read returns to the second hostdevice over the particular link.

In at least one example, the memory access link protocol is differentfrom the GPIO link protocol, the GPIO link protocol includes a physicallayer and a link layer, a physical layer of the memory access linkprotocol is based on the physical layer of the GPIO link protocol, and alink layer of the memory access link protocol is based on the link layerof the GPIO link protocol.

In at least one example, the first host device includes a first CPU of acomputing device and the second host device includes a second CPU of thecomputing device.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

In the foregoing specification, a detailed description has been givenwith reference to specific exemplary embodiments. It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense. Furthermore, the foregoing use of embodiment andother exemplarily language does not necessarily refer to the sameembodiment or the same example, but may refer to different and distinctembodiments, as well as potentially the same embodiment.

What is claimed is:
 1. An apparatus comprising: a memory buffer deviceto: identify a sequence of read returns to be sent to a host device overan interface, wherein the interface is to comprise a transactionalbuffered memory interface, and the sequence is to comprise at least afirst read return to a first read request and a second read return to asecond read request; encode a tracker identifier of the second readreturn in the first read return; send the first read return with thetracker identifier of the second read return to the host device; andsend the second read return to the host device after the first readreturn is sent.
 2. The apparatus of claim 1, wherein the interface is tocomprise a link compliant with a buffered memory link protocol.
 3. Theapparatus of claim 2, wherein the buffered memory link protocolincorporates a physical layer and link layer of a general purposeinput/output (GPIO) interconnect protocol.
 4. The apparatus of claim 1,wherein the memory buffer device is to receive a sequence of readrequests from the host device and the sequence of read requestscorrespond to the sequence of read returns.
 5. The apparatus of claim 4,wherein a final one of the read returns in the sequence is to be encodedto indicate a dummy value for an early transaction identifier.
 6. Theapparatus of claim 1, wherein the first read return is to furthercomprise a tracker identifier of the first read return.
 7. The apparatusof claim 6, wherein the first read return is to comprise a headeredpacket and the second read return is to comprise a headerless packet. 8.The apparatus of claim 7, wherein the first read return is to comprisethe headered packet based on the first read return being the first readreturn in the sequence and the subsequent read returns in the sequenceare comprise headerless packets.
 9. The apparatus of claim 7, whereinheadered packets each comprise a set of flits to comprise one headerflit and four data flits and headerless packets each comprise a set ofthree data flits.
 10. The apparatus of claim 7, wherein the trackeridentifier of the second read return is to be encoded in a first dataflit of the first read return and the tracker identifier of the firstread return is to be encoded in the header flit of the first readreturn.
 11. The apparatus of claim 1, wherein the first read request andfirst read return correspond to a first transaction and the second readrequest and second read return correspond to a second transaction. 12.The apparatus of claim 1, wherein the sequence further comprises a thirdread return to correspond to a third read request and the second readreturn is to be encoded with a tracker identifier of the third readreturn.
 13. An apparatus comprising: a host device to comprise: atransmitter to: send a first read request of a first transaction to abuffer device; and send a second read request of a second transaction tothe buffer device; and a receiver to receive a first read return fromthe buffer device, wherein the first read return is to comprise a firsttracker identifier to indicate that the first read return corresponds tothe first read request, and is to further comprise an early secondtracker identifier to indicate that a next read return is to correspondto the second read request.
 14. The apparatus of claim 13, wherein thereceiver is further to receive the next read return from the bufferdevice.
 15. The apparatus of claim 14, wherein the host device is tofurther comprise a host controller to map the first read return to thefirst read request based on the first tracker identifier and map thesecond read return to the second read request.
 16. The apparatus ofclaim 13, wherein the host device comprises a central processing unit.17. A system comprising: a first host device; a buffer device connectedto the first host device using a buffered memory link compliant with amemory access link protocol; one or more memory devices connected to thebuffer device; and a second host device connected to the second hostdevice using a particular link compliant with a general purposeinput/output (GPIO) link protocol, wherein the buffer device is to:identify a sequence of read returns to be sent to the first host deviceover the buffered memory link, wherein the sequence is to comprise atleast a first read return to a first read request and a second readreturn to a second read request; encode a tracker identifier of thesecond read return in the first read return; send the first read returnwith the tracker identifier of the second read return to the first hostdevice; and send the second read return to the first host device afterthe first read return is sent.
 18. The system of claim 17, wherein thehost device is further to forward data of at least one of the first andsecond read returns to the second host device over the particular link.19. The system of claim 18, wherein the memory access link protocol isdifferent from the GPIO link protocol, the GPIO link protocol comprisesa physical layer and a link layer, a physical layer of the memory accesslink protocol is based on the physical layer of the GPIO link protocol,and a link layer of the memory access link protocol is based on the linklayer of the GPIO link protocol.
 20. The system of claim 19, wherein thefirst host device comprises a first CPU of a computing device and thesecond host device comprises a second CPU of the computing device.